Abstract:

A polarization-selective diffractive optical element includes a liquid
crystal polymer film supported by a substrate. The liquid crystal polymer
film includes an array of pixels, each pixel encoded with a fixed liquid
crystal director such that each liquid crystal director is aligned in a
common plane perpendicular to the liquid crystal polymer film and
provides a predetermined pattern of out-of-plane tilts. A size of the
pixels in the array and the predetermined pattern are selected such that
the liquid crystal polymer film forms a phase hologram for diffracting
light polarized parallel to said common plane and a zeroth order
diffraction grating for light polarized perpendicular to the said common
plane. The non-etched and flat phase hologram is suitable for a wide
range of applications.

Claims:

1. A polarization-selective diffractive optical element comprising:a
substrate;an alignment layer disposed on the substrate; anda liquid
crystal polymer film disposed on the alignment layer, the liquid crystal
polymer film including a plurality of liquid crystal directors aligned
parallel to a first plane, the first plane perpendicular to a surface of
the liquid crystal polymer film, an out-of-plane tilt of the plurality of
liquid crystal directors varying with transverse spatial coordinate in a
predetermined pattern, the predetermined pattern selected such that the
liquid crystal polymer film forms a polarization-selective phase
hologram,whereby linearly polarized light having a first polarization is
transmitted through first and second spatially distinct regions of the
liquid crystal polymer film with a relative phase delay to provide a
non-zeroth order diffraction output, and linearly polarized light having
a second polarization is transmitted through the first and second
spatially distinct regions with substantially zero relative phase delay
to provide a zeroth order diffraction output, the first polarization
parallel to the first plane, the second polarization orthogonal to the
first polarization, the first region including a first liquid crystal
director, the second region including a second liquid crystal director,
the first and second liquid crystal directors having different
out-of-plane tilts.

2. A polarization-selective diffractive optical element according to claim
1, wherein the predetermined pattern is selected such that the liquid
crystal polymer film includes a plurality of pixels, each pixel encoded
with a single liquid crystal director alignment.

3. A polarization-selective diffractive optical element according to claim
2, wherein the predetermined pattern includes a finite number of
different out-of-plane tilt angles, the finite number greater than two
and less than sixty-five.

4. A polarization-selective diffractive optical element according to claim
1, wherein the phase hologram includes a grating vector that is parallel
or orthogonal to the first plane.

5. A polarization-selective diffractive optical element according to claim
1, wherein the phase hologram includes a grating vector that is at an
oblique angle to the first plane.

6. A polarization-selective diffractive optical element according to claim
1, wherein the predetermined pattern is selected to form a non-periodic
phase mask for providing aberration correction in an optical pick-up
unit.

7. A polarization-selective diffractive optical element according to claim
1, wherein the predetermined pattern is selected to form a grating for
redirecting a beam of light reflected from an optical disc disposed in an
optical pick-up unit away from an input optical path.

8. A polarization-selective diffractive optical element according to any
of claim 1, wherein the predetermined pattern is selected to form a
grating for providing polarization discrimination in an external cavity
laser.

9. A polarization-selective diffractive optical element according to claim
2, wherein a size of the pixels and the predetermined pattern are
selected to form a grating for providing beam steering for light having
the first polarization.

10. A polarization-selective diffractive optical element according to
claim 9, wherein the liquid crystal polymer film is disposed in series
with a second liquid crystal polymer film, the second liquid crystal
polymer film including a second plurality of liquid crystal directors
aligned parallel to a second plane, the second plane perpendicular to a
surface of the second liquid crystal polymer film, an out-of-plane tilt
of the second plurality of liquid crystal directors varying with
transverse spatial coordinate in the predetermined pattern.

11. A polarization-selective diffractive optical element according to
claim 10, wherein the first plane and the second plane are substantially
parallel.

12. A polarization-selective diffractive optical element according to
claim 11, wherein the liquid crystal polymer film is oriented relative to
the second liquid crystal polymer film such that an unpolarized beam of
light incident on the polarization-selective diffractive optical element
is converted to two substantially parallel beams of light having
orthogonal polarizations.

13. A polarization-selective diffractive optical element according to
claim 1, wherein the substrate includes at least one of a waveplate and a
reflective surface.

14. A method of fabricating a polarization-selective diffractive optical
element comprising:irradiating an alignment layer at oblique angle
through a photo-mask with linearly polarized UV light;coating a liquid
crystal layer on the irradiated alignment layer, the liquid crystal layer
including a liquid crystal polymer precursor;irradiating the liquid
crystal layer to form a liquid crystal polymer film, the liquid crystal
polymer film including a plurality of liquid crystal directors aligned
parallel to a first plane, the first plane perpendicular to a surface of
the liquid crystal polymer film, an out-of-plane tilt of the plurality of
liquid crystal directors varying with transverse spatial coordinate in a
predetermined pattern, the predetermined pattern selected such that the
liquid crystal polymer film forms a polarization-selective phase
hologram,whereby linearly polarized light having a first polarization is
transmitted through first and second spatially distinct regions of the
liquid crystal polymer film with a relative phase delay to provide a
non-zeroth order diffraction output, and linearly polarized light having
a second polarization is transmitted through the first and second
spatially distinct regions with substantially zero relative phase delay
to provide a zeroth order diffraction output, the first polarization
parallel to the first plane, the second polarization orthogonal to the
first polarization, the first region including a first liquid crystal
director, the second region including a second liquid crystal director,
the first and second liquid crystal directors having different
out-of-plane tilts.

15. A method according to claim 14, wherein the photo-mask comprises one
of a variable transmission mask and a variable size aperture mask.

16. A polarization-selective diffractive optical element comprising:a
substrate;a liquid crystal layer supported by the substrate in the form
of a thin planar film having an array of pixel regions that have been
encoded with a finite number of differing liquid crystal director
alignments,wherein the liquid crystal director alignment in each pixel
region is substantially uniform and permanent throughout the
pixel,wherein the liquid crystal director alignment in each pixel region
lies in a common plane perpendicular to a surface of the substrate in
order to impart a phase delay to linearly polarized light incident on the
array that is polarized parallel to the said plane of the liquid crystal
directors and to have substantially no phase delay effect on linearly
polarized light incident on the array that is polarized perpendicular to
the plane of the liquid crystal directors, andwherein an arrangement of
phase delays in the pixel array, pixel size, and pixel shape, are
predetermined so that the liquid crystal layer provides non-zeroth order
diffractive output for light polarized parallel to the plane of the
liquid crystal directors and zeroth-order diffractive output for light
polarized perpendicular to the plane of the liquid crystal directors.

17. An optical pick-up unit comprising:a light source for emitting
linearly polarized light having a first polarization;a collimating lens
for collimating the linearly polarized light;an objective lens for
focusing the collimated linearly polarized light onto an optical disc;a
quarter-wave plate disposed between the collimating lens and the
objective lens for providing quarter-wave retardance such that light
reflected from the optical disc is transmitted towards the first lens as
linearly polarized light having a second polarization, the second
polarization orthogonal to the first polarization; anda
polarization-selective diffractive optical element disposed between the
collimating lens and the quarter-wave plate, the polarization-selective
diffractive optical element including a substrate, an alignment layer
disposed on the substrate, and a liquid crystal polymer film disposed on
the alignment layer, the liquid crystal polymer film including a
plurality of liquid crystal directors aligned parallel to a first plane,
the first plane perpendicular to a surface of the liquid crystal polymer
film, an out-of-plane tilt of the plurality of liquid crystal directors
varying with transverse spatial coordinate in a predetermined pattern,
the predetermined pattern selected such that the liquid crystal polymer
film forms a polarization-selective phase hologram,wherein the
polarization-selective diffractive optical element is disposed such that
the first polarization is polarized perpendicular to the first plane and
such that the polarization-selective phase hologram provides zero order
diffraction for the linearly polarized light having the first
polarization and non-zeroth order diffraction for the linearly polarized
light having the second polarization, the non-zeroth order diffraction
providing a beam deflection sufficient to redirect the linearly polarized
light having the second polarization away from the light source and
towards a detector.

[0004]Diffraction gratings and more complex thin holograms, encoded onto
programmable liquid crystal (LC)-based spatial light modulators (SLMs),
have been actively researched as a way to alter the wavefront of an
optical beam. For example, these LC/SLMs may be used for adaptive-optic
phase correction, in a synthetic phase array, or in a telecommunication
beam steering switch. The LC/SLMs are based typically on either a
transmissive or reflective type micro-display panel in order to provide
the small pixel pitch requirement. LCs with both in-plane (e.g., such as
in-plane-switching (IPS) using nematic LC and ferroelectric LC) and
out-of-plane (e.g., planar or parallel aligned (PA) and vertical aligned
(VA) nematic LC) rotation of LC director are utilized. The ferroelectric
LC (FLC) will be polarization insensitive if the hologram is configured
with two phase levels. Polarization insensitivity can be important for
systems where the light source has unknown or scrambled polarization,
such as for a beam-steering switch used in telecommunication networks. On
the other hand, since out-of-plane switching nematic LCs (e.g., PA and VA
nematic LC) are known to be polarization sensitive, holograms recorded
onto these LC/SLMs generally require a known polarization. Accordingly,
these types of LC holograms are typically only useful in optical systems
and instrumentation where the light sources are polarized.

[0005]Although programmable thin holograms encoded onto LC/SLMs are very
versatile, these active components are not cost effective for many
applications. In addition, these programmable thin holograms are known to
provide relatively small steering angles. For example, a state-of-the art
LC on Silicon (LCoS) panel may have less than 10 μm pixel pitch, which
at a wavelength of 0.5 μm and utilizing a minimum of two pixels per
grating period, provides a maximum beam deflection angle of about 1.4
degrees. All other programmable hologram output (e.g., termed the replay)
will have even smaller deflection angles.

[0006]Nevertheless, there has been interest in forming passive diffraction
gratings or holograms based on these active device. For example, in U.S.
Pat. No. 6,304,312, a diffraction grating is formed by injecting liquid
crystal monomer between two transparent substrates, each of which is
coated with an alignment layer. In one example, the alignment layer is
uniform and the diffraction grating is effected by applying a voltage to
patterned electrodes provided on the transparent substrates. In another
example, the diffraction grating is effected with a patterned alignment
layer (e.g. patterned using a photolithography technique). After the
liquid crystal layers are aligned, they are then polymerized and/or
cross-linked to fix the alignment. Note that the liquid crystal polymer
pixels in this reference are limited to having either homeotropic
alignment (i.e., perpendicular to the substrate) or planar alignment
(i.e., parallel to the substrate). The resulting binary grating (e.g.,
having a pitch of about 8 μm) is reported to provide only about forty
percent diffraction efficiency.

[0007]More recently, patterned photo-alignment layers having an even
smaller pixel pitch (e.g., 1 μm or shorter) have been proposed. For
example, in U.S. Pat. No. 7,375,784 a micro-patterned alignment layer is
disclosed. While the alignment layer is limited to having only
homeotropic alignment (i.e., perpendicular to the substrate) and planar
alignment (i.e., parallel to the substrate), the liquid crystal may be
aligned with a range of out-of-plane angles. More specifically, local
alignment of the liquid crystal is stated to be determined by the average
areas of underlying homeotropic alignment and planar alignment regions.
Unfortunately, since the alignment of the liquid crystal is related to an
average of different regions it cannot be patterned with precision and
thus, is not suitable for many applications.

[0008]In fact, in order to optimize precision and cost effectiveness, most
applications requiring passive holograms use diffractive optical elements
with physical steps. Unfortunately, the etching and/or molding processes
used to form these diffractive optical elements are relatively complex
and time consuming. In addition, the surface relief structure generally
requires complex optical thin-film coating processes to protect the
delicate structures.

[0009]It would be advantageous to provide a method of fabricating thin
film gratings or holograms that is relatively simple, low cost, and/or
that is suitable for a wide range of applications.

SUMMARY OF THE INVENTION

[0010]The instant invention relates to a method of forming diffraction
gratings and/or holograms with thin liquid crystal polymer layers. In one
embodiment a thin liquid crystal polymer is formed on an alignment layer,
which has been irradiated with linearly polarized light at non-normal
incidence through a photo-mask. In this embodiment, the photo-mask is
patterned such that the light is incident on different areas of the
alignment layer with different energy densities. Advantageously, each
region of the alignment layer irradiated with a different energy density
provides a different out-of-plane tilt angle in the overlying region of
the liquid crystal polymer coated thereon. Accordingly, a hologram having
a plurality of tilt-angles between zero and ninety degrees is easily
formed with precision. As a result, relatively complex hologram
structures are easily designed for a wide range of applications. In
addition, since the liquid crystal polymer film is coated on a single
substrate and patterned without etching and/or molding, the resulting
holograms are flat and can be provided at low cost.

[0011]The instant invention also relates to diffractive optical elements
formed using these non-etched and flat (NEF) holograms, wherein the
liquid crystal (LC) out-of-plane tilt varies with transverse spatial
coordinate in a predetermined manner. In one embodiment, the resulting
NEF thin diffractive optical element has the LC director in each pixel of
the hologram aligned along a given azimuthal plane. The plane containing
the LC director distribution is also the tilt plane. Only light rays
polarized along the tilt plane are affected by the variable amount of
retardance encoded continuously or in a pixelated manner. The variable
amount of retardance is a manifestation of variable optical path length
modulation as a function of transverse spatial coordinate. Conversely,
the light rays polarized along a direction orthogonal to the tilt plane
sample only the ordinary index of refraction regardless of the LC direct
tilt. The variable optical path length modulation is absent and this
orthogonal polarization essentially experiences a zeroth-order grating.
In other words, these high-efficiency gratings are
polarization-selective. For a first linear polarization, the incident
light rays are allowed to diffract to non-zeroth order locations while
for a second orthogonal linear polarization, the incident light rays are
not diffracted and their light energy is preserved within the zeroth
diffraction order.

[0012]The instant invention is also related to the use of the NEF
diffraction gratings and/or holograms in various applications.

[0013]In accordance with one aspect of the instant invention there is
provided a polarization-selective diffractive optical element comprising:
a substrate; an alignment layer disposed on the substrate; and a liquid
crystal polymer film disposed on the alignment layer, the liquid crystal
polymer film including a plurality of liquid crystal directors aligned
parallel to a first plane, the first plane perpendicular to a surface of
the liquid crystal polymer film, an out-of-plane tilt of the plurality of
liquid crystal directors varying with transverse spatial coordinate in a
predetermined pattern, the predetermined pattern selected such that the
liquid crystal polymer film forms a polarization-selective phase
hologram, whereby linearly polarized light having a first polarization is
transmitted through first and second spatially distinct regions of the
liquid crystal polymer film with a relative phase delay to provide a
non-zeroth order diffraction output, and linearly polarized light having
a second polarization is transmitted through the first and second
spatially distinct regions with substantially zero relative phase delay
to provide a zeroth order diffraction output, the first polarization
parallel to the first plane, the second polarization orthogonal to the
first polarization, the first region including a first liquid crystal
director, the second region including a second liquid crystal director,
the first and second liquid crystal directors having different
out-of-plane tilts.

[0014]In accordance with another aspect of the instant invention there is
provided a method of fabricating a polarization-selective diffractive
optical element comprising: irradiating an alignment layer at oblique
angle through a photo-mask with linearly polarized UV light; coating a
liquid crystal layer on the irradiated alignment layer, the liquid
crystal layer including a liquid crystal polymer precursor; irradiating
the liquid crystal layer to form a liquid crystal polymer film, the
liquid crystal polymer film including a plurality of liquid crystal
directors aligned parallel to a first plane, the first plane
perpendicular to a surface of the liquid crystal polymer film, an
out-of-plane tilt of the plurality of liquid crystal directors varying
with transverse spatial coordinate in a predetermined pattern, the
predetermined pattern selected such that the liquid crystal polymer film
forms a polarization-selective phase hologram, whereby linearly polarized
light having a first polarization is transmitted through first and second
spatially distinct regions of the liquid crystal polymer film with a
relative phase delay to provide a non-zeroth order diffraction output,
and linearly polarized light having a second polarization is transmitted
through the first and second spatially distinct regions with
substantially zero relative phase delay to provide a zeroth order
diffraction output, the first polarization parallel to the first plane,
the second polarization orthogonal to the first polarization, the first
region including a first liquid crystal director, the second region
including a second liquid crystal director, the first and second liquid
crystal directors having different out-of-plane tilts.

[0015]In accordance with another aspect of the instant invention there is
provided a polarization-selective diffractive optical element comprising:
a substrate; a liquid crystal layer supported by the substrate in the
form of a thin planar film having an array of pixel regions that have
been encoded with a finite number of differing liquid crystal director
alignments, wherein the liquid crystal director alignment in each pixel
region is substantially uniform and permanent throughout the pixel,
wherein the liquid crystal director alignment in each pixel region lies
in a common plane perpendicular to a surface of the substrate in order to
impart a phase delay to linearly polarized light incident on the array
that is polarized parallel to the said plane of the liquid crystal
directors and to have substantially no phase delay effect on linearly
polarized light incident on the array that is polarized perpendicular to
the plane of the liquid crystal directors, and wherein an arrangement of
phase delays in the pixel array, pixel size, and pixel shape, are
predetermined so that the liquid crystal layer provides non-zeroth order
diffractive output for light polarized parallel to the plane of the
liquid crystal directors and zeroth-order diffractive output for light
polarized perpendicular to the plane of the liquid crystal directors.

[0016]In accordance with another aspect of the instant invention there is
provide an optical pick-up unit comprising: a light source for emitting
linearly polarized light having a first polarization; a collimating lens
for collimating the linearly polarized light; an objective lens for
focusing the collimated linearly polarized light onto an optical disc; a
quarter-wave plate disposed between the collimating lens and the
objective lens for providing quarter-wave retardance such that light
reflected from the optical disc is transmitted towards the first lens as
linearly polarized light having a second polarization, the second
polarization orthogonal to the first polarization; and a
polarization-selective diffractive optical element disposed between the
collimating lens and the quarter-wave plate, the polarization-selective
diffractive optical element including a substrate, an alignment layer
disposed on the substrate, and a liquid crystal polymer film disposed on
the alignment layer, the liquid crystal polymer film including a
plurality of liquid crystal directors aligned parallel to a first plane,
the first plane perpendicular to a surface of the liquid crystal polymer
film, an out-of-plane tilt of the plurality of liquid crystal directors
varying with transverse spatial coordinate in a predetermined pattern,
the predetermined pattern selected such that the liquid crystal polymer
film forms a polarization-selective phase hologram, wherein the
polarization-selective diffractive optical element is disposed such that
the first polarization is polarized perpendicular to the first plane and
such that the polarization-selective phase hologram provides zero order
diffraction for the linearly polarized light having the first
polarization and non-zeroth order diffraction for the linearly polarized
light having the second polarization, the non-zeroth order diffraction
providing a beam deflection sufficient to redirect the linearly polarized
light having the second polarization away from the light source and
towards a detector.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]Further features and advantages of the present invention will become
apparent from the following detailed description, taken in combination
with the appended drawings, in which:

[0018]FIG. 1a is a side-view of index indicatrix projection of a prior art
LC hologram with azimuthal angle distribution;

[0019]FIG. 1b is a plan-view of the LC hologram shown in FIG. 1a;

[0020]FIG. 2a is a side-view of index indicatrix of an LC hologram with
polar angle distribution in accordance with one embodiment of the present
invention;

[0033]FIG. 11 is a schematic diagram of a prior art OPU including a
non-periodic phase mask that functions as a polarization-selective
wavefront aberration compensator;

[0034]FIG. 12 is a schematic diagram of a prior art non-periodic
phase-mask including of annular regions, wherein the optical axis of the
uniaxial A-plate is oriented uniformly across all pupil positions;

[0035]FIG. 13 shows the phase profile of the phase mask illustrated in
FIG. 12 along the XZ cross-section;

[0036]FIG. 14 is a schematic diagram of an OPU including a
polarization-selective non-etched flat (NEF) holographic optical element
that functions as a polarization-selective wavefront aberration
compensator, in accordance with one embodiment of the instant embodiment;

[0037]FIG. 15 shows the polar-angle tilt profiles of a LC hologram
according to one embodiment of the present invention (top plot shows the
out-of-plane polar-angle tilt profiles within each phase-mask region for
two cases of maximum LC director tilt angles, whereas the middle and
bottom plots show the required index indicatrix projection along the
cross-sectional planes of XZ and YZ, respectively);

[0038]FIG. 16 is a schematic diagram of a prior art OPU including a
surface-relief structure (SRS) and/or planar hologram as a
polarization-selective beam steering device;

[0039]FIG. 17 is a schematic diagram of an OPU including a
polarization-selective non-etched flat holographic optical element as the
polarization-selective beam steering device, in accordance with one
embodiment of the instant invention;

[0040]FIG. 18 shows a three-wavelength periodic grating structure steering
the light beams to the first order;

[0041]FIG. 19 shows the diffraction angular spectra of a three-wavelength
BD/DVD/CD system, for the phase profiles shown in FIG. 18, where each
encoding pixel is 1 μm;

[0042]FIG. 20 is a schematic diagram showing part of an OPU including
polarization-selective non-etched flat holographic optical elements as
the polarization-selective wavefront aberration compensator and the beam
steering device;

[0043]FIG. 21 is a schematic diagram showing part of an OPU including a
polarization-selective non-etched flat holographic optical element as the
polarization-selective beam steering device for tapping off beamlets in
disc-tracking and objective lens focusing, control and feedback;

[0044]FIG. 22 is a schematic diagram of a thin LC hologram incorporated
with a quarter-wave plate in accordance with one embodiment of the
instant invention;

[0045]FIG. 23 is a schematic diagram of a thin LC hologram mounted on a
reflective substrate or on a reflective layer on a transparent substrate,
in accordance with one embodiment of the instant invention;

[0046]FIG. 24 is a schematic diagram of a flat LC hologram used for
polarization-selective beam-steering;

[0047]FIG. 25 is a schematic diagram of a prior art Rochon polarizer
utilizing negative uniaxial birefringent crystal such as calcite or
α-BBO;

[0048]FIG. 26 is a schematic diagram of an external-cavity laser utilizing
plano-plano reflectors, wherein the laser includes a laser crystal, a
flat LC hologram polarization-selective beam-steering device, and a
second harmonic generation crystal;

[0049]FIG. 27 is a schematic diagram of a dual-stage flat LC hologram beam
steering device wherein the selected polarization in both stages are
parallel;

[0050]FIG. 28 is a schematic diagram of a dual-stage beam-displacer with
flat LC hologram beam steering devices wherein the selected polarization
in both stages are parallel and the diffraction angle sense is opposite;

[0051]FIG. 29 is a schematic diagram of a dual-stage flat LC hologram beam
steering device wherein the selected polarizations in both stages are
orthogonal and both polarization beamlets are deflected to the opposite
angular directions;

[0052]FIG. 30 is a schematic diagram of a dual-stage flat LC hologram beam
walk-off device wherein the selected polarizations in both stages are
orthogonal and both polarization beamlets are deflected to the same
angular direction;

[0058]FIG. 36 is a plan view of (a) a first stage LC hologram, (b) second
stage QWP, and (c) a third stage LC hologram with orthogonal steering;

[0059]FIG. 37 is a plan view of (a) a first stage LC hologram with
horizontal grating vector, (b) orthogonal linear polarization output of
first stage LC hologram and their resolved components parallel and
orthogonal to a new coordinate basis, and (c) a second stage LC hologram
with orthogonal steering (vertical grating vector) and rotated
tilt-plane;

[0060]FIG. 38 is a schematic diagram of a prior art Babinet-Soleil's
compensator with a movable top birefringent wedge; and,

[0061]FIG. 39 is a schematic diagram of a variable retarder with a LC film
having a polar-angle distribution, in accordance with one embodiment of
the instant invention.

[0062]It will be noted that throughout the appended drawings, like
features are identified by like reference numerals.

DETAILED DESCRIPTION

[0063]A prior-art thin liquid crystal (LC) hologram structure is
illustrated in FIG. 1, which is a thickness cross-sectional view along
the grating vector. The grating vector is the plane where the light is
dispersed by diffraction effect. It is also the pixelation direction for
a 1D grating or hologram. The hologram 5 includes a substrate 1, onto
which an array of pixels 10 having varying azimuthal LC director
orientations is disposed. Four discrete azimuthal LC director
orientations are shown as 11, 12, 13 and 14. More specifically, the
projection of the LC index indicatrix onto the plane of drawing
(XZ-plane) is shown. Pixel 11 has its projected director aligned parallel
to the X-axis, whereas pixel 13 has its projected director aligned
parallel to the Y-axis. The other two states, pixels 12 and 14 have their
projected directors contained within the XY plane but non-parallel to
both the X- and Y-axes. The hologram element 5 also includes AR coating
stacks 2 and 3 to aid transmission efficiency.

[0064]In operation, a light ray incident along the Z-axis 20 is spatially
sampled by the hologram, wherein the spatial phase encoding causes the
beam to be steered at an angle 22 as output ray 21. It is noted that,
depending on the hologram design, other diffraction orders, in addition
to 21 may also be present at the output. The output may also contain the
zeroth order (undiffracted) light, as a result of diffraction
inefficiency.

[0065]A key feature of this prior-art LC hologram is that all the pixels
are configured as either A-plates (i.e., an optical retardation element
having its extraordinary axis oriented parallel to the plane of the
layer) or O-plates (i.e., an optical retardation element having its
extraordinary axis oriented obliquely to the plane of the layer), with
variable LC director azimuthal orientations. In other words, there is no
variation in the out-of-plane tilt of the LC directors. Referring to FIG.
1b, the variation in LC director azimuthal orientation across several
pixels is shown. The four discrete pixel states, 11, 12, 13 and 14, have
their LC directors aligned approximately at 0, 45, 90 and 135 degrees,
respectively, relative to the hologram vector 25.

[0066]Note that the hologram configuration illustrated in FIGS. 1a and 1b
is polarization sensitive. In particular, it is expected that for one
circular polarization the four pixel states represent progressively
advancing phase distribution and therefore the light will be steered
towards one direction. For the opposite handedness of circular
polarization, the same four pixel encoding represent progressively
delaying phase distribution and the light is steered to the symmetric
orders. However, while this LC grating is polarization sensitive, it is
not polarization selective. The diffraction effect cannot be completely
turned off, even if one has complete control over the incoming
polarization. The two circular Eigen-polarizations always replay to
symmetric patterns. Other polarizations (linear or in general elliptical)
are combinations of the two circular states and hence replay to some
mixture of the two circular polarization outputs. No input polarization
selection is able to preserve all the light power in the undiffracted
order.

[0067]Referring to FIGS. 2a and 2b, there is shown a
polarization-selective hologram in accordance with one embodiment of the
instant invention. FIG. 2a is a cross-sectional view along the grating
vector 45. The hologram 30 includes a substrate 31 onto which a
continuously-varying or a pixelated LC layer 40 is disposed. The hologram
element 30 also includes AR coating stacks 32 and 33 to aid transmission
efficiency. Polarization-selectivity is provided by aligning the LC
directors in different pixels with variable amount of out-of-plane tilts,
while maintaining a uniform azimuthal orientation. Four discrete pixels
states 41, 42, 43 and 44 are shown with approximately 0, 33.6, 53.1 and
90 degree of out-of-plane tilt angles, respectively. At a nominal
λ=400 nm, the ordinary index no and extraordinary index
ne of refraction values are 1.61 and 1.75, respectively, such that
these four pixels give rise to [0 -0.0461 -0.0921 -0.1382] phase
difference per unit length relative to the A-plate configured pixel 41.
For an LC film thickness of approximately 2.17 μm, these four pixels
provide for [0, π/2, π, 3π/2] phase encoding, which are the
optimal discrete states for four-level phase-only holograms.

[0068]In operation, X-polarized input light 50 incident along the Z-axis
is steered to the main diffraction order 51 with a deflection angle of
52. It is noted that, depending on the hologram design, other diffraction
orders in addition to 51 may also be present at the output. The output
may also contain the zeroth order (undiffracted) light, as a result of
diffraction inefficiency. With the orthogonal linear polarization input
(for example Y-polarization), the LC hologram 30 presents no optical path
length modulation. This light is not diffracted and is contained in the
zeroth-order output. In other words, by configuring the LC hologram as an
array of variable-tilt encoded pixels, the thin hologram is made
polarization-selective. With one linear polarization, the hologram
diffracts. With the orthogonal linear polarization, the hologram is
highly transparent.

[0069]Referring to FIG. 2B, the plane of tilt 46 is parallel to the
hologram vector 45. The series of dark arrows indicate the effective
in-plane birefringence. More generally, the hologram vector dictates the
direction at which the light ray is diffracted whereas the plane of tilt
dictates the linear polarization that sees the LC hologram. The linear
polarization that is diffracted is aligned parallel to the tilt-plane.
The linear polarization orthogonal to the tilt plane is undiffracted.

[0070]Referring to FIG. 2c, there is shown a polarization-selective
hologram in accordance with another embodiment of the instant invention.
In this embodiment, the hologram vector 45a of the LC hologram 35 is
parallel to the X-axis, but the tilt plane 46a is chosen with an
azimuthal offset 57 to the hologram vector. As a result, light rays
polarized parallel to the tilt plane 46a are diffracted along a plane
parallel to the grating vector 46a. Note that while the out-of-plane
angle of the LC director in each pixel varies between pixels, the
azimuthal angle of the LC director is the same between pixels.

[0071]Referring to FIG. 3, there is shown a system for fabricating a flat
non-etched polarization-selective diffractive optical element (e.g., a
hologram) in accordance with one embodiment of the invention. The optical
setup 60 includes a mount for supporting the device under fabrication 65,
a linearly polarized ultra-violet (LPUV) light exposure system 70, and a
photo-mask 75. The device under fabrication 65 includes a substrate 66
onto which a linear photo-polymerizable (LPP) alignment layer 67 is
deposited. The LPUV exposure system 70 includes a UV light source 71, a
collimating lens 72, and a UV polarizer 73. The photo-mask 75 is
patterned to provide varying levels of light to the alignment layer in a
predetermined manner. In particular, the photo-mask 75 is patterned to
provide varying levels of energy density to the alignment layer as a
function of transverse spatial coordinate. In one embodiment, the
photo-mask 75 is a variable transmission mask. In another embodiment, the
photo-mask 75 is a variable size aperture mask.

[0072]In operation, the light source 71 provides LPUV light at an oblique
angle to the surface of the substrate 66. In this embodiment, the light
source is shown to be tilted relative to the horizontal substrate. In
other embodiments, the substrate is tilted relative to the light source.
The non-normal LPUV light incidence and its energy density dose induce a
change in the alignment layer 67 that causes the LC director in a
subsequently deposited LCP pre-cursor layer to be aligned at an oblique
angle (tilted out of the plane of the substrate at some azimuthal angle).
In this embodiment, the UV polarizer 73 is oriented to transmit, with
high transmission, UV light polarized parallel to the plane of drawing
(e.g., which is the plane of incidence). Depending on the chemistry of
the LPP material, this configuration will typically result in the LC
director of the subsequently deposited LCP layer to be aligned in an
azimuthal plane that is parallel or orthogonal to the LPUV plane of
incidence. The actual out-of-plane tilt of the LC director is dependent
on the LPUV energy density dose delivered to the LPP alignment layer 67.
Since the photo-mask 75 provides various energy densities to the
alignment layer 67 in a predetermined pattern a spatially variable tilt
LCP film, which has variable in-plane retardance, results. Although the
out-of-plane tilt of the LC director varies in a predetermined manner
across the film, the azimuthal angle of the LC directors is constant as
for example, illustrated in FIGS. 2b and 2c. For example, in one
embodiment the LC director is aligned homogeneously along a single
azimuthal plane but with variable tilt angles. Once the LPP layer is
exposed to LPUV in this manner, then a thin layer of liquid crystal
polymer precursor is coated on the alignment layer. This layer is then
exposed to UV light (e.g., which does not have to be polarized) to
cross-link the LCP precursor and fix the LC directors at the
predetermined oblique angles. Accordingly, this method allows diffraction
gratings and more complex thin holograms to be encoded on thin LCP
layers, supported by a single substrate, to provide stable diffractive
optical elements that are suitable for a wide range of applications. In
addition, since the LCP film need only be supported by a single substrate
the thin NEF polarization-selective diffractive element is easily
integrated with other optics.

[0073]Note that this fabrication technique has been described with
reference to a LCP precursor, which is preferably cross-linked with a
subsequent UV irradiation to convert it to LCP. In general, the LCP layer
may be formed using any of the LPP and liquid crystalline compounds known
in the art, the latter of which may be polymerized and/or cross-linked
with UV irradiation and/or thermally. For example in one embodiment, the
LPP layer is formed by spin-coating a 2 wt % solution of a LPP in
cyclopentanone on a glass substrate (e.g., for 60 seconds at 3000 RPM) to
obtain a 50 nm thick alignment layer. In other embodiments, the LPP
layers are formed using another coating method such as wire-coating,
gravur-coating, slot-coating, etc. LPP layers, which often include
cinnamic acid derivatives and/or ferulic acid derivatives, are well known
in the art. In accordance with the instant invention, the LPP layer will
be of the type to generate an out-of-plane tilt in the subsequently
applied LC or LCP layers. Various compounds suitable for forming the LPP
layer are available from Rolic (Allschwil, CH). In one embodiment, the
LPP coated glass is baked for a predetermined time (e.g., 5 minutes) at a
predetermined temperature (e.g., 180 degrees) before being LPUV
irradiated through the photo-mask. In one embodiment, the LPP is
irradiated in a two step process. In the first step, the layer is exposed
to linearly polarized light without the photo-mask (e.g., through a
standard aperture, to set the lowest tilt-angle at all locations). In a
second step, the layer is exposed to the linearly polarized light through
the photo-mask (e.g., to set the higher tilt-angles at select locations
corresponding to the transmitting areas of the photo-mask). In this
embodiment, the total energy density (i.e. dose) delivered will be higher
at those regions exposed in the first and second irradiation steps, as
compared to those regions only exposed in the first irradiation step. In
general, the required energy density and wavelength of illumination will
be dependent on the LPP material. In general, the energy density will be
typically between 30-300 mJ/cm2, while the wavelength range will be
typically between 280 and 365 nm. In the embodiment shown above, the
photo-mask is patterned to provide varying amounts of energy. In other
embodiments, the photo-mask is moved relative to the substrate to provide
the varying amounts of energy. In each case, the incident angle of LPUV
will be typically between 20 and 60 degrees. As discussed above, the
irradiated LPP layer is used as an orientation layer for the subsequently
coated LCP layer. In one embodiment, the LCP layer is formed from liquid
crystalline material that includes a liquid crystal polymer precursor.
LCP precursor materials, which for example may include a cross-linkable
diacrylate nematic liquid crystalline compound, are well known in the
art. In accordance with the instant invention, the LCP material will be
of the type that will appropriately respond to the tilt inducing LPP
layer. Various LCP precursor compounds suitable for forming the LCP layer
are available from Rolic (Allschwil, CH). In one embodiment the LCP
precursor layer is spin-coated on the LPP layer as a 15 wt % solution in
anisole. In other embodiments, the LCP layers are formed using another
coating method such as wire-coating, gravur-coating, slot-coating, etc.
The resulting LLP/LCP device is then typically baked (i.e annealed) for a
predetermined time to promote good alignment of the LCP to the LPP
alignment layer. Advantageously, the subsequent photochemical
cross-linking of the LCP layer is believed to provide improved
reliability under high power illumination and short wavelength laser
exposure.

[0074]An example of a response curve of LPUV exposure dose for a LPP/LCP
system is shown in FIG. 4. The solid line plots the in-plane
birefringence as a function of the LPUV dose density. In the case of
creating a variable retarder, the LPUV dose density corresponds to a
transverse spatial coordinate. The effective in-plane birefringence is
obtained by taking the projection of the full LC indicatrix onto the
device plane. The decreasing effective birefringence with increasing LPUV
energy density indicates that the out-of-plane LC director tilt increases
with LPUV energy density. The LC director tilt is plotted as a dashed
line in FIG. 4.

[0075]In general, the photo-mask 75 will be patterned in dependence upon
the intended application. In one embodiment, the photo-mask 75 is
patterned to provide varying energy densities to the alignment layer 67
in a pixelated manner. In other embodiments, the photo-mask 75 is
patterned to provide varying energy densities to the alignment layer 67
in a continuously graded manner. In one embodiment, the pixels are
periodic (e.g., at regular intervals). In another embodiment, the pixels
are non-periodic (e.g., random or in a predetermined pattern).
Advantageously, the use of the photo-mask 75 allows the LCP layer to be
patterned with a large number of phase profile levels and with increased
precision. In one embodiment, the photo-mask 75 is patterned to provide
two levels of phase profile. In another embodiment, the photo-mask 75 is
patterned to provide more than two levels of phase profile. In general,
most applications will require at least 4 levels of phase profile in
order provide reasonable diffraction efficiency. The level of phase
profile on diffraction efficiency is described below.

[0076]The simplest thin hologram is a regular grating, where the grating
period has as many pixels as there are distinct phase levels. A
phase-only grating is also called a kinoform. The diffraction expression
predicts that a m-level grating produces p-order diffraction output with
an efficiency, ηpm, of

where λ is the wavelength of illumination and Λ is the
grating period (i.e., the pitch). Taking a small angle approximation
(e.g., sin(θ)˜θ) and a Fourier transform lens of focal
length f,

Δ θ λ ##EQU00003##

where Δx is the spatial translation of the diffracted output, and w
is the pixel pitch, the expression above can be generalized as,

Δ σ λ Δ Δ στ λ
##EQU00004##

for 1D and 2D hologram replay, respectively, where (σ,τ)
represents the fractional hologram main diffraction order location within
the zeroth-order replication region, and fλ/w is the physical size
centered at the optical axis (e.g., see K. L. Tan et al., "Dynamic
holography for optical interconnections. II. Routing holograms with
predictable location and intensity of each diffraction order," J. Opt.
Soc. Am. A, 18(1), pp. 205-215, 2001). The fractional orders lie within
±1/2 replication region. In this notation, the spatial sampling and
replication (i.e., artifacts of hologram recording device and hologram
replay) is decoupled from the hologram generation. For each grating
recording, unless all m level phase steps are present in the grating and
the total available phase range is 2π*(m-1)/m, and each encoding cell
has 100% pixel-fill duty cycle ratio, the diffraction efficiency of the
first replay order will be lower than predicted in eq. (1).

[0078]Assuming that the LC hologram recording and replay operation is
idealized (lossless), the ideal first order diffraction efficiencies for
several phase-only gratings are given as follows:

[0079]Accordingly, for a highly efficient hologram replay, the number of
distinct phase levels should be greater than 8.

[0080]A four-level phase-only hologram is illustrated in FIG. 5. The top
plot represents a side view showing the LC director orientation along the
tilt-plane. The bottom plot, which is an out-of-plane polar angle tilt
profile, shows the discrete tilt steps required to realize the lossless
quaternary phase hologram. This configuration is frequently termed -1/4
fractional replay, because the light is mainly steered to 1/4 distance to
the left of the zeroth order within the central replay replication. This
σ=-1/4 polarization-selective periodic phase mask (e.g., a
grating), which exhibits asymmetric replay, diffracts light having a
linear polarization input parallel to the plane of drawing and is
transparent to the orthogonal linear polarization. As discussed above,
this four-level hologram is expected to yield a maximum of 81%
diffraction efficiency in the first diffraction order. In order to
increase the diffraction efficiency, more phase levels are required.
Without loss of generality, a single encoding element can be represented
as an LC director inclined at an angle with respect to the Z-axis and
contained within the XZ plane. Referring to FIG. 6, the LC director 81
forms a polar angle offset 82 θc with the Z-axis. The LC
director out-of-plane tilt 83 θt is given by
π/2-θc. From the quadratic equations describing the index
ellipsoid, the in-plane na and out-of-plane nc effective
indices are represented by the projection onto the XY-plane 80 and
projection along the Z-axis 85. These effective indices are given by,

θλ θ λ θ λ θλ θ
λ θ λ ##EQU00005##

where ne(λ) and no(λ) are the dispersion of the
extraordinary and ordinary indices of the uniaxial material. In terms of
advancing phase, relative to an A-plate aligned pixel (θt=0),
Eq. (6) gives a non-linear increase of phase ramp with increasing of
out-of-plane tilt. The phase difference relative to an A-plate configured
pixel (i.e., na(θt;λ)-ne(λ)) is plotted
in FIG. 7. From the plot, an encoding pixel, aligned with the LC tilt at
˜56.7°, yields a phase difference per unit length of -0.1.
In other words, a 2 μm pixel height provides for 200 nm phase advance
relative to the A-plate pixel. This gives the required π phase step at
λ=400 nm. A linear phase ramp, as is often required in a
multiple-level phase hologram, can be configured from the phase per unit
length versus tilt angle profile.

[0081]Referring to FIGS. 8 and 9a-c, there are shown various embodiments
of thin, polarization-selective holograms having two phase levels. FIG. 8
shows a binary LC hologram, encoded as alternating A-plate/C-plate
pixels. The top plot represents a side view showing the LC director
orientation along the tilt-plane. The bottom plot shows the out-of-plane
polar angle tilt profile. This σ=±-1/2 polarization-selective
periodic phase mask (e.g., a grating) gives a symmetric replay and
diffracts light having a linear polarization input parallel to the plane
of drawing and is transparent to the orthogonal linear polarization. As
discussed above, this two-level hologram is expected to yield a maximum
of 40.5% diffraction efficiency in the first diffraction order. In order
to increase the diffraction efficiency, more phase levels are required.
Note that this hologram gives the highest frequency encoding capability
at the given minimum pixel size. With the same LC material use in the
calculations described above, this hologram is only 1.45 μm thick,
sufficient to create a π phase step with the full LC birefringence. An
image of this binary LC hologram may be presented as a series stripes, as
shown in FIG. 9a. In this embodiment, the bright stripes represent 0
phase pixels whereas the dark-stripes represent the π phase pixels. In
other embodiments, the bright stripes represent π phase pixels whereas
the dark-stripes represent the 0 phase pixels (i.e., the two LC polar
angle tilts have an optical path difference of π phase). Referring to
FIG. 9b, there is shown an embodiment of a 2D beam steering hologram.
This checker-board hologram steers the light to the maximum spatial
frequency locations for both X and Y directions. FIG. 9c shows an
embodiment of a crossed Dammann grating. This hologram steers light three
times as far in the Y-direction as it steers light along the X-direction.
In all three binary hologram examples, it has been assumed that the
hologram operates in the scalar diffraction domain. The effective indices
for TE and TM waves are not impacted by the pixelation. Rather, the plane
of tilt within the hologram encoding, which is uniform over the entire
hologram and may or may not coincide with any of the 1D or 2D steering
plane, dictates the linear polarization for which the hologram diffracts
and the orthogonal linear polarization for which the hologram is
transparent.

[0082]One application of a polarization-selective hologram in accordance
with one embodiment of the instant invention is in an optical pick-up
unit (OPU). For example, consider the prior art OPU system illustrated in
FIG. 10. The OPU 100 includes an array of semiconductor laser sources
110, the output of which are spatially multiplexed by an array of
Polarization Beam Combiner (PBC) cubes 130, collimated by a lens system
160, folded by a leaky mirror 140, and imaged (focused) onto a single
"pit" area on the rotating disc media 150 via a second objective lens
element 161. The leaky mirror allows a small fraction (e.g. 5%) of the
incident beam energy to be tapped off and focused onto a monitor
photodiode (PD) 175 via another lens element 165. The array of
semiconductor laser sources 110 is shown as 3 discrete laser diodes (LD),
including a first LD 111 at λ=400 nm, a second LD 112 at
λ=660 nm, and a third LD 113 at λ=780 nm. The outputs from
the array of LDs 110 are substantially linearly polarized (e.g., `S`
polarized with respect to the PBC hypotenuse surface). The linearly
polarized beams are passed through an array of low-specification
polarizers 120, which protect the LD sources 110 from unwanted feedback
should the orthogonal polarization ray be reflected towards the laser
cavities by the first 131, second 132, and/or third 133 PBCs in the array
130.

[0083]In operation, the main beam in each of the LD sources is steered
along the common path 180 towards the information layer (IL) within the
disc media 150. Prior to reaching the achromatic quarter-waveplate (QWP)
145, the beam is substantially linearly polarized. The achromatic QWP 145
transforms the linear polarization (LP) into circular polarization (CP),
the handedness of which is dependent on the orientation of the optic axis
of the achromatic QWP 145 (e.g., for a given S- or P-polarization input).
In this example, where `S` polarization is input to the achromatic QWP
145, left-handed circular polarization will result if the optic axis
(i.e., slow-axis) of the achromatic QWP 145 is aligned at 45°
counter clockwise (CCW) with respect to the P-plane of the PBC (e.g.,
with the assumption of intuitive RH-XYZ coordinate system while looking
at the beam coming to the observer). When the rotating disc media 150 is
a pre-recorded compact disc (CD) or digital versatile disc (DVD), where
there is a physical indentation of a recorded pit, the optical path
length difference between a pit and its surrounding "land", at 1/6 to 1/4
wave, causes destructive interference (e.g., at least partial) and
reduces the light detected by the main photodiode 170 positioned at the
second port of the PBC cube array 130. In the absence of a pit, there is
no destructive interference and the light will be effectively transformed
by the achromatic QWP 145, upon double-passing there through, from the
initially S-polarization to P-polarization, such that substantially the
same light power returns towards the PBC cube array 130.

[0084]When the rotating disc medium 150 includes more than one information
layer per single side of disc, such as a DVD dual-layer (DL) disc, the
separation between the two IL layers is typically between 20-30 μm in
order to reduce coherent crosstalk when accessing the disc. Although the
objective lens 161 is readily adjusted to focus onto the required IL
depth, this refocusing causes spherical aberrations. For the DVD legacy
system with an objective lens having about 0.6 numerical aperture (NA)
and utilizing 650 nm of illumination wavelength, the change in IL depth
may not be critical. However, in other DL formats (e.g., Blu-ray (BD) and
high definition (HD)-DVD), the corresponding increase in NA (e.g., 0.85
NA for BD) and decrease in wavelength of illumination (e.g.,
approximately 405 nm) causes spherical aberrations of roughly 200-300
mλ when the high NA objective lens is refocused onto a second IL
depth (e.g., for dual-layer disc format having an approximately 20 μm
spacer layer with ˜1.5 index of refraction). There are various ways
to reduce this aberration. For example, it is common to mechanically
adjust the elements in a compound objective lens and/or adjusting the
position of the collimation lens to alter the vergence of the entrance
beam to the objective lens. Alternatively, various non-mechanical
aberration correction schemes have been proposed.

[0085]Referring to FIG. 11 there is shown one example of an OPU including
non-mechanical aberration correction (i.e., which is similar to the OPU
system proposed in U.S. Pat. No. 6,947,368). In this figure, elements
similar to those described in FIG. 10 are referred to with like numbers.
In addition to the optical components described in FIG. 10, the OPU 200
in FIG. 11 also includes an actively switched LC cell 210 and a
non-periodic phase-mask 220, which are inserted in the parallel beam
section between the collimating lens 160 and the objective lens 161. Note
that the collimating lens 160 is positioned before the PBC 131 rather
than after.

[0086]In operation, the beam that is deflected 90-degrees by the PBC 131
is S-polarized with respect to the PBC hypotenuse plane. This beam is
passed through the active LC cell 210 such that one of the two orthogonal
linear polarizations is output (e.g., S-polarization and P-polarization
with respect to the PBC hypotenuse (also parallel to Y-axis and X-axis,
respectively)). Depending on the LC mode of operation, the electrical
driving state (on or off) for producing a given output (for example
S-polarization shown in FIG. 11) can be different. With an 90-degree
twisted nematic LC cell, the cell has to be driven off to produce the
same polarization output as it is the input. With a VA nematic LC cell,
the same polarization output as in the input is obtained without driving
the cell. Yet other LC modes such as FLC and IPS nematic LC will require
appropriate voltage driving to either alter the polarization or maintain
the polarization of incoming light beam.

[0087]In FIG. 11, S-polarized radiation is presented to the phase mask
220. The phase-mask 220 includes a series of physical steps etched into a
birefringent layer or a birefringent substrate. In general, fabrication
of these physical steps is achieved using photo-lithography and dry/wet
etching techniques. In one embodiment, the etched steps of the phase mask
220 are exposed to air. In another embodiment, the phase mask 220 is
formed by filling the air gaps obtained from patterning and etching with
an isotropic material, which may or may not possess the same index as one
of the birefringent medium principal indices of refraction. In this
embodiment, the air/birefringent phase mask 220 has a uniform slow-axis
orientation aligned parallel to the P-plane (e.g., the plane of drawing
in FIG. 11) and the step height is configured as 2π phase jump for air
and ne index of refraction. Hence, when S-polarization is
transmitted through the phase-mask 220 it imparts a phase-modulation.
When P-polarization (not shown) is allowed to come through the LC cell
210, the phase mask is inactive.

[0088]When the objective lens is at the nominal focus (e.g., to be
focussed on the inner information layer 154 at depth ˜100 μm),
the LC cell 210 transmits P-polarized light (not shown) that passes
through the quarter waveplate 145 and is reflected back and focussed on
the detector 170 via lens 163. When reading/writing to the outer
information layer 153 (e.g., at ˜80 μm depth), the objective
lens is refocused. Refocusing without changing the vergence of the beam
coming to the objective lens causes spherical aberrations. In order to
reduce the spherical aberrations, the LC cell 210 is used to transmit
S-polarization when the focal position is changed from the nominal value.
The S-polarization samples the no index in the phase mask 220, to
produce the desired wavefront.

[0089]Note that the phase mask 220 is a surface-relief structure (SRS)
including a series of annular zones. For example, consider the prior art
phase mask 250 illustrated in FIG. 12, which has a central
reference-phase zone. The birefringent material has its optic axis 252
aligned along the X-axis. The incoming S-polarized beam 253 is aligned to
the Y-axis. Where the incoming beam samples the air segment within the
phase mask, it represents a phase advance relative to the central annular
zone. It is the opposite in the focusing beam when the focal distance is
brought from ˜100 μm to ˜80 μm IL depth. The phase
profile across the XZ cross-section 251 is shown in FIG. 13. The example
indicated close to 1.2π of total phase range is required to reduce the
rms wavefront aberrations, as a result of the focal change, from
approximately 200 mλ to ˜40 mλ. The corrected wavefront
aberration is diffraction limited at the operating wavelength.

[0090]Referring again to FIG. 11, the etched phase mask 220 in combination
with the LC switch 210 allows two polarization states to be selectively
corrected for wavefront aberrations dependent on which information layer
is being accessed on disc. For a given nominal objective lens focal
(either to the inner or the outer information layer), the complement
phase profile of the associated aberrations when refocusing is
implemented can be encoded onto the phase mask. By switching the LC cell
output polarizations, each information layer is accessed with wavefront
aberrations contained within the diffraction limit.

[0091]Unfortunately, since the phase mask 220 is typically fabricated by
etching a birefringent element, it is generally considered to be a
relatively expensive optical element. In accordance with one embodiment
of the instant invention, a photo-cured LCP layer encoded with a
predetermined phase profile (e.g., formed by patterning the effective
in-plane birefringence using the oblique photo-alignment technique
described with reference to FIG. 3) is used in a non-mechanical
aberration correction scheme.

[0092]Referring to FIG. 14, a schematic diagram of an OPU 300 in
accordance with one embodiment of the instant invention is shown. In this
figure, elements similar to those described in FIGS. 10 and 11 are
referred to with like numbers. Note that a non-etched and flat (NEF) LC
phase mask 310 is provided instead of the conventional etched phase mask
220.

[0093]In operation, a collimated beam of light is coupled as
S-polarization 231 into the common path through the reflection port of a
PBC 131. The LC switch 210 converts the S-polarization to the orthogonal
P-polarization 232 (e.g., with respect to the PBC hypotenuse). This
P-polarization is parallel to the plane of drawing and is also parallel
to the uniform azimuthal orientation of the thin NEF LC phase mask 310.
The NEF phase mask has a variable LC out-of-plane tilt, as a function of
the pupil position. The effective extraordinary index changes with LC
director tilt. Hence, the optical path length is tailored by configuring
the LC tilt. In the active phase correction case, the P-polarization
samples the phase of each encoding pixel differently, in a manner
required to create the complementary phase profiles associated to
changing the nominal focal point of the objective lens, when a second
information layer is to be accessed, at a different depth than the first
information layer where the objective lens has been configured
aberration-free. In the non-active phase correction case with the second
linear polarization output from the LC cell (not shown), the beam samples
the no index regardless of the tilt within each encoding pixel. The
LC hologram is a transparent zeroth-order grating and no phase
preconditioning of the beam is effected.

[0094]This preconditioned beam then traverses a quarter-waveplate 145
which converts the first linear polarization 232 into a first circular
polarization 233. Upon reflection at the information layer, a second
(opposite handedness) circular polarization 234 is obtained. This beam is
again converted to the second linear polarization 235 by the
quarter-waveplate 145. The phase correction is active in the first pass
but the phase correction is inactive in the second pass and vice versa,
depending on the LC cell switching. The second pass phase correction does
not matter since the beam is not refocused tightly on the way to the
photodetector.

[0095]The LC director tilt profile across the pupil coordinate is shown in
FIG. 15. Plot (a) shows the out-of-plane LC director tilt for two cases
of maximum tilts: 70 and 90 degrees, in order to generate the
discrete-step phase profile as shown in FIG. 13. The calculation
wavelength is 400 nm and at this wavelength, the ordinary index no
and extraordinary index ne of refraction values are 1.61 and 1.75,
respectively. The required LC film thickness is approximately 1.94 μm
and 1.74 μm for creating a 1.2π maximum phase range with 70-degree
and 90-degree maximum tilt, respectively. This film is very thin and it
has a constant physical thickness across the aperture. The polar angle
distribution across the pupil gives in the phase correction function. The
LC director (also the slow-axis) is aligned along a common plane for
example along the XZ-plane in the example given. The LC director profile
for several discrete pixels, along the XZ plane, also the tilt plane, is
illustrated in plot (b) of FIG. 15. Again the central annular zone has a
reference phase provided by the ne index of the LC film. Progressing
outwards from the pupil center, the phase initially advances, by sampling
an effective index, between the ne and no of the LC film until
zone #7 where the LC is aligned at the maximum tilt (either 70-degree or
90-degree). Beyond this annular zone, the phase difference to the central
zone decreases progressively towards the limit of the pupil by decreasing
the LC tilt. Along a plane orthogonal to the tilt plane, the projection
of the effective LC index indicatrix is shown in plot (c) of FIG. 15.
Since this is a vertical plane, the longer indicatrix pixel gives a lower
effective retardance for normal incidence rays and hence advancing phase
versus the shorter indicatrix pixels.

[0096]In the embodiments described with reference to FIG. 14 a
polarization-selective hologram in accordance with the instant invention
is used in a non-mechanical aberration correction scheme. Advantageously,
the non-periodic mask 310 is uniform in layer thickness across the clear
aperture. When the optic axis of the uniaxial LC material is aligned to
an oblique tilt, as well as the required planar and homeotropic
alignment, the phase mask 310 may be used with the liquid crystal cell
210 to provide a polarization-selective wavefront phase correction or
total transparency. Advantageously, the polarization-selective phase mask
310 works with linear polarization, which is conveniently provided by the
laser diode light sources with a high polarization purity. Accordingly,
the polarization-selective phase mask 310 does not need to be positioned
after the quarter waveplate 145, wherein the lack of purity of circularly
polarized light may reduce diffraction efficiency.

[0097]In other embodiments, a polarization-selective hologram in
accordance with the instant invention is used as a beam steering element
in an OPU. For example, consider the prior art OPU system 400 illustrated
in FIG. 16, wherein a polarization-selective periodic grating 410
provides a function similar to the PBS cubes 130 illustrated in FIG. 10.
In this system 400, which is similar to that proposed in Japanese Pat.
Appl. JP-A-2001-174614 and US Pat. Appl. No. 2006/0239171, the grating
410 is used to angularly (and spatially) separate the return beam from
the optical disc from the radiation coming from the laser diodes. In
particular, the grating 410 utilizes the large optical rotary power
dispersion near the reflection band edge of a cholesteric liquid crystal
and near the absorption band etch of an organic dye to preferentially
diffract a required circular polarization to +1 order (e.g., also ±1
orders for binary periodic grating) while being transparent to the
orthogonal circular polarization (e.g., there is little to no
diffraction, and light appears mostly in the zeroth order).

[0098]The OPU system 400 includes a co-packaged laser diode and detector
module 305. The laser diode section of the module 305 launches a
divergent beam towards a collimating lens 162, which produces a parallel
beam of a first linear polarization 231 (i.e., which for, illustrative
purposes shown to be orthogonal to the plane of drawing). The linear
polarization 231 is converted to a first circular polarization 233 upon
passing through a quarter-waveplate 145. For a preferred cholesteric
helical twist having the opposite handedness as the circular polarization
input, this circular polarization 233 is not impacted by the periodic
grating 410. The beam is then focused on the disc media 150 by a high NA
objective lens 161. More specifically, the beam is focussed on an
information layer 153 in the disc, which is covered with a protective
layer 152 and disposed on a substrate 151. Reflection off the disc
changes the handedness of the circular polarization such that the
reflected beam 234 has a second circular polarization that is opposite to
the first. Since this second circular polarization has the same
handedness as the cholesteric helical twist, the beam is steered by the
cholesteric/isotropic periodic grating 410 on return pass. When the beam
is transmitted through the quarter-waveplate 145 for a second time, and a
second linear polarization results 236 (e.g., which is orthogonal to the
first linear polarization). Depending on the grating pitch and wavelength
of operation, the return beam is deflected by an angle 320, according to
the grating equation (2). The angular deflection is converted to spatial
offset by lens 162, resulting in a beam offset Δx 321.

[0099]In other words, the polarization-selective periodic grating 410
functions as a holographic beam splitter, which in a forward propagating
direction does not provide beam steering so as to preserve beam energy
transmitted to the disc 150, and in a backward propagating direction
provides beams steering so as to separate the information-bearing beam
from the input beam. While this scheme is promising, there are several
drawbacks related to the polarization-selective periodic grating 410.
First, the wavelength-selectivity of the periodic grating 410 means that
only one wavelength of a multiple-wavelength OPU system (e.g., the
BD/DVD/CD system illustrated in FIG. 10) can be configured to be
diffracting or non diffracting at a given circular polarization. As a
result, in order for the holographic beam-splitter to work in a BD/DVD/CD
system, it must be designed with three grating layers. This add costs and
weight which counters the aim of reducing component size. Note that the
wavelength-selectivity is likely related to the fact that the grating
works near the cholesteric reflection band edges. A second drawback of
the polarization-selective periodic grating 410 is that it works with
circularly polarized light. In an OPU system, circularly polarized light
is only available between the quarter-waveplate 145 and the disc 150. In
addition, the efficacy of the grating 410 is dependent on the purity of
the circularly polarized light generated after the quarter-waveplate. A
third drawback is that the periodic grating 410 is typically fabricated
with by patterning and etching a substrate, and often by filling the
etched substrate. As discussed above, these fabrication techniques are
often time consuming and relatively high cost. In addition, when the
etched surface is filled with another material, it is likely that the
refractive index of the filling material will not match the refractive
index of the birefringent grating across all wavelength bands of
interest. In the non-diffracting case, the cholesteric pixels and the
isotropic pixels do not typically have the same index values and a
complete suppression of the unwanted circular polarization at all
operating wavelengths may not be possible. A fourth drawback is that the
achievable grating resolution is generally limited. For example, consider
the first example provided in US Pat. Appl. No. 2006/0239171. In this
example, where the cholesteric LC has a rather high linear birefringence
(e.g., Δn=0.2), the 4×4 matrix modeled circular birefringence
is approximately 0.04 (e.g., π phase step at λ=660 nm and
physical step height of the binary grating of 8.8 μm). This large step
impacts the achievable grating resolution. For example, to create a 1
μm pixel width, a 9:1 aspect ratio (height to width ratio) is
required, which makes the etching step difficult. For higher-efficiency
multi-level phase gratings, the required phase range may approach 2π,
requiring even larger aspect ratios. In other words, the prior art is
limited in practice to binary phase gratings having coarse grating
resolutions, which are not efficient and have small steering angles.

[0100]Referring to FIG. 17, a schematic diagram of an OPU 500 in
accordance with an embodiment of the instant invention is shown, wherein
a polarization-selective periodic LC diffraction grating 510 is provided
to replace the polarization-selective periodic grating 410 used in FIG.
16. This non-etched and flat (NEF) LC diffraction grating 510 utilizes a
variable tilt LCP film to create an array of variable retarder elements.
The slow-axes of all grating pixels are aligned in the same azimuthal
plane, but with different amounts of polar angle tilt.

[0101]In operation, a co-packaged laser diode and detector module 305
launches a divergent beam towards a collimating lens 162, which produces
a parallel beam of a first linear polarization 231 (e.g., which for
illustrative purposes is shown orthogonal to the plane of drawing). This
linear polarization 231 is orthogonal to the tilt-plane of the
polarization-selective LC hologram 510. Since the LC hologram is
transparent to this linear polarization, the transmitted light is
contained in the zeroth order and is converted to a first circular
polarization 233 upon passing through a quarter-waveplate 145. The beam
is then focused on the disc media 150 by high numerical aperture (NA)
objective lens 161. Reflection at the disc 150 changes the handedness of
the circular polarization and upon return, beam 234 has the second
(opposite) handedness of beam 233. The second circular polarization then
passes through the quarter-waveplate 145 for a second time to provide a
second linear polarization 236. This second linear polarization is
steered by the polarization-selective LC periodic grating 510 on return
pass. Depending on the grating pitch and wavelength of operation, the
return beam is deflected by an angle 320, according to the grating
equation (2). The angular deflection is converted to spatial offset by
lens 162, resulting in a beam offset Δx 321.

[0102]In contrast to the prior-art circular-polarization-selective grating
410 discussed above, the polarization-selective LC periodic grating 510
is selectively a hologram and a transparent device, depending on the
state of linear polarization input. In contrast to the narrow-band
characteristics of a near band-edge cholesteric alternating with
isotropic-filling grating 410, the polarization-selective LC periodic
grating 510 is operational over a relatively broad band.

[0103]As an example, simple grating structures intending to steer light to
only the first diffraction order for three discrete wavelength of Blu-ray
Disc(BD) or High-definition (HD)-DVD/DVD/CD OPU system is illustrated in
FIG. 18. The LC hologram tilt profile is configure as a lossless
phase-only grating at the intermediate wavelength of 660 nm. The phase
ramp is configured by varying the LC tilt in successive pixels. At the
design wavelength of 660 nm, the 16-level phase grating spans 0 to
15π/8 and each encoding pixel is assumed to have a width of 1 μm.
With the LC material described above, the LC film thickness is 5.9 μm,
if a full range of 0 to 90 degree tilt is usable. At the longer 780 nm
wavelength, the natural dispersion of the LC mixture results in less than
2π phase ramp. The hologram diffraction at this wavelength will have
zeroth order undiffracted light output. Conversely, the increase
birefringence at the short 400 nm wavelength coupled with the shorter
full-wave optical path difference requirement results in nearly 4π of
phase ramp at λ=400 nm. This means that first order diffraction
angles will be approximately the same for all three discrete wavelengths
(e.g., at λ=400 nm, the wavelength is nearly half that of
λ=780 nm, but its spatial grating period is also nearly half that
of the NIR grating). The angular spectrum of the thin LC grating for a
polarization input parallel to the LC tilt plane is show in FIG. 19. The
design wavelength channel has a first order diffraction efficiency (DE)
of approximately 98%. The other two light channels had a first order DE
of approximately 88%. In addition, when the input polarization is
orthogonal to the LC tilt plane, the LC hologram behaves as a zeroth
order grating at any wavelength of illumination. The zeroth order grating
may be lossless if polarization purity is assured and external AR losses
are excluded.

[0104]In US Pat. Appl. No. 2006/0239171, the overall thickness of their
binary cholesteric/isotropic grating was approximately 10 μm (e.g.,
which is similar to the above described 5.9 μm). However, the
symmetric replay meant that the first order DE is at best 40%. In some
other wavelength bands, the reported theoretical DE is less than 10%, due
to the phase encoding inefficiency of the dye-based material. With the
low circular birefringence in the prior-art techniques, coupled with the
requirement to perform photolithography and etching, the aspect-ratio
constraint will not permit more than several phase steps. Furthermore, a
single grating fabricated this way will not permit simultaneous steering
of multiple channels because the circular birefringence is derived close
to the absorption/reflection band edges.

[0106]Referring to FIG. 20, a schematic diagram of an OPU 600 in
accordance with an embodiment of the instant invention is shown, wherein
a first NEF thin hologram 510, which is a periodic grating, functions as
holographic beam splitter and a second NEF thin hologram 310, which is a
non-periodic phase mask, pre-conditions the wavefront of a
reading/writing beam when the objective lens is refocused onto a
non-design information layer depth. In this embodiment, the first 510 and
second 310 NEF thin holograms function as described with reference to
FIGS. 17 and 14, respectively. The disc 150 is shown to include a first
information layer 153 and a second information layer 154, which are
disposed on a substrate 151 and separated with a spacer layer 155.

[0107]Referring to FIG. 21, a schematic diagram of an OPU 700 in
accordance with another embodiment of the instant invention is shown,
wherein a NEF thin hologram 710 is used to tap off a small amount of the
return beam. In commercial OPU systems, a small amount of the return beam
is frequently tapped in order to track the spiral grooves on the disc
media, astigmatism induced by disc warpage, and/or disc placement at an
angle versus the read/write beam. The tap-off beam is often imaged to
multiple element arrayed-detector. The actual signal beam is allowed to
go through to the main photodiode. In such a scenario, the LC hologram
design may seek to contain the main beam within the zeroth order and
allow a small fraction (say 5%) of the light to one or more replay
orders. The OPU system 700 launches one or more channels of laser diode
output to the common path via the reflecting port of the PBC 131. The
S-polarization is not diffracted by the polarization-selective LC
hologram 710 in the first pass. On the return pass, the polarization is
converted to one that is parallel to the tilt plane of the LC hologram.
The LC hologram is now designed and encoded to replay a large zeroth
order. Accordingly, a co-packaged detector array 705 includes the main
photodiode 721 for detecting the main signal and one or more auxiliary
photodetectors 722 for detecting the tracking beam(s). This predominantly
zeroth-order replay can be accomplished, for example, by deliberately
providing inadequate phase range. The ideal phase range (e.g., the
difference of the first to the last phase steps available for pixel
encoding) is 2π*(m-1)/m, where m is the number of phase levels. For
example, lossless binary and quartemary phase-only holograms require π
and 1.5π phase ranges. The zeroth order undiffracted light (e.g., the
sum of the geometric center replay) and all high replication centers is
given by,

Φ Φ ##EQU00006##

[0108]where Φ is the total phase range available for encoding up to m
levels of phase steps, sinc(x)=sin(x)/x and sinc(0)=1. For a binary phase
hologram, the DC undiffracted light fraction is cos2(Φ/2). A
binary hologram may be the most suitable for tracking purpose in an OPU,
where the symmetric replay orders may be useful in detecting geometric
skewing and most of the light has to be contained in the zeroth order
(i.e., where the diffracted orders do not have to be efficient). For
example, if 90% of the light is to be retained as the zeroth order, a
binary grating only has to have a phase modulation of ˜37 degrees.
Under ideal encoding condition, including equal pixel widths of 0 and
37-deg, phase steps, the ±1st orders can be expected to yield
about 4% light output for tracking purpose. In other embodiments, the
polarization-selective LC hologram may be configured to replay the signal
beam to the first diffraction order and the tracking beams to other
replay orders.

[0109]In the embodiments described above, the polarization-selective thin
LC holograms provide a phase map for one linear polarization and appear
transparent for the orthogonal linear polarization. For example, in one
embodiment, the phase map is an aberration correcting non-periodic
wavefront map. In another embodiment, the phase map is a periodic grating
or hologram that provides beam steering. In these embodiments, the
polarization-selective thin LC holograms are supported by a single
substrate mounted separately in the corresponding OPU systems. As
described above, it is also possible for the polarization-selective thin
LC holograms to be supported by another optical element. For example,
referring to FIG. 21 the polarization-selective thin LC hologram 710 may
be integrated with the quarter waveplate 145.

[0110]Referring to FIG. 22, a compound polarization-selective device 1100
in accordance with one embodiment of the instant invention includes a
substrate 901 onto which a LC hologram 1010 is disposed. The LC hologram
1010 includes several pixels 1011, 1012, 1013, 1014 patterned to effect
beam steering. The LC tilt plane is aligned parallel to XZ plane, such
that the linear polarization parallel to the XZ plane is beam steered
whereas linear polarization parallel to the Y-axis is not affected. On
the opposite side of the substrate 901, a quarter-waveplate 1120 having
one or more layers of birefringent materials is disposed. The slow- and
fast-axis of the QWP 1120 are typically aligned at ±45 degree with
respect to the X or Y-axis. As a result, the indicatrix 1121 shown is a
projection of the full indicatrix onto the plane of drawing. The device
1100 also includes optical AR coatings 902 and 903 to enhance the overall
transmittance. In the embodiment described with reference to FIG. 22, the
QWP is integrated on the opposite side of the substrate. In another
embodiment, the QWP is integrated one the same side of the substrate as
the LC hologram, either above or underneath the LC hologram layer.
Regardless of the configuration, when this compound element 1100 is used
in an OPU, such as that described with reference to FIG. 21, it is
preferably positioned such that the LC hologram is within the
linear-polarization segment of the OPU.

[0111]In operation, a light beam incident parallel to the Z-axis 920 is
spatially modulated by the encoded phase profile in 1010. The exiting
beam deviates from the specular direction by a small angle. The beam is
passed through the QWP 1120, which converts the linear polarization to a
circular polarization. This beam then exits the assembly as 921 having an
angle offset of 922.

[0112]Referring to FIG. 23, a compound polarization-selective device 1200
in accordance with another embodiment of the instant invention is shown.
This compound device 1200 includes a LC hologram 1010 that is disposed on
a reflector 1203, which is in turn disposed on a transparent substrate
901. The opposite side of the LC hologram is coated with an AR coating
902.

[0113]In operation, an incoming light beam 920 is transmitted through the
device 1200 such that wavefront is sampled in the first pass towards the
reflector, and a second time on its return from the reflector.
Accordingly, the required phase range is half that of a transmissive LC
grating device. The output beam 1221 is steered towards the angular
direction having the denser pixels (i.e., pixels having A-plate or
ne index of refraction within a grating period). For an identical LC
hologram configuration (e.g., same pixel size, phase range, phase
encoding at each pixel and wavelength of operation) as the transmissive
LC grating device 500 illustrated in FIG. 17, device 1200 will steer
through twice as large diffraction angle. Note, however, that the
diffraction efficiency may not be maintained, since the double pass gives
an effect of having fewer phase steps.

[0114]In the embodiments described above, the NEF thin LC holograms
function as linear polarization-selective beam steering devices. When
configured as a single-spot high efficiency grating replay, the LC
hologram transmits the ordinary wave unaffected and steers the
extraordinary wave by a small angle. The angle offset is approximately
the ratio of the wavelength and grating pitch length (eq. 2). Within the
visible and NIR wavelength bands and with practical micron-size pixels, a
16-pixel grating can be configured to steer the main beam to about 2
degrees at >98% efficiency (sin-1(0.55/16) as steering angle).
This quantum of walk-off angle is useful in many applications.

[0115]Referring to FIG. 24, a high efficiency LC grating is used as a
standalone beam-steering device 1300. The device 1300 includes a
transparent substrate 1319 for supporting a LC grating film 1310. The LC
grating film 1310 includes a plurality of pixels with tailored phase
profile effected by arranging the LC out-of-plane tilt as required. One
of the phase pixels 1311 is shown to have C-plate optical symmetry.
Another of the pixels 1312 is shown to have A-plate optical symmetry.
There intervening pixels (e.g., between 1311 and 1312) are shown to be
configured as pixels with O-plate optical symmetry.

[0116]In operation, an unpolarized light beam of light 1320 is incident on
the left side of the device 1300. The unpolarized beam of light 1320
includes equal amounts of light polarized parallel to the LC tilt plane
and light polarized orthogonal to the LC tilt plane, as indicated by
1321. As the unpolarized beam of light 1320 passes through the LC grating
1310, the linear polarization orthogonal to the LC tilt plane samples the
o-wave index of the grating pixels and is transmitted unaffected. This
o-beam exits as 1330 having a linear polarization perpendicular to the
tilt plane 1331. On the other hand, the linear polarization parallel to
the LC tilt plane samples the effective e-wave index of the grating
pixels. The spatial phase profile of the grating 1310 creates a
differential-phase wavefront, which steers the e-wave to non-zero output
angles along a direction parallel to the grating vector plane. The e-wave
1340 exits the LC grating device 1300 having a linear polarization 1341
parallel to the tilt-plane. The steering angle is given by 1345. It is
noted that in general the tilt-plane does not have to be parallel to the
grating vector plane. The tilt-plane selects the diffracted linear
polarization whereas the grating vector selects the plane of diffraction.

[0117]Notably, this single-stage LC hologram device 1300 is functionally
equivalent to a prior-art Rochon polarizer made of two crystal wedges. A
schematic diagram of a Rochon polarizer is shown FIG. 25. The crystal
polarizer 1350 includes a first wedge 1360 which is aligned with its
optic axis parallel to the nominal beam direction and a second wedge 1361
which is aligned with its optic axis orthogonal to the plane of drawing.
A light ray input 1370 having polarization components parallel and
orthogonal to the plane of drawing samples the ordinary index of
refraction while propagating through the first wedge unchanged. At the
wedge boundary, the linear polarization parallel to the plane of drawing
continues to sample the ordinary index in the second wedge and therefore
exits the polarizer unaffected (without change in polarization and beam
direction). The other linear polarization which is orthogonal to the
plane of drawing samples the extraordinary index in the second wedge.
With the use of negative uniaxial crystal materials, the resultant drop
in index values means the ray is refracted away from the normal line to
the wedge boundary. The second linear polarization is steered to an angle
while exiting the polarizer. If the wedges are made of calcite crystals,
having no and ne indices of [1.66 and 1.49] at λ=550 nm,
the large birefringence is calculated to provide about 7-degrees of beam
steering within the second wedge which is equivalent to about 10 degree
in air. Notably, the NEF diffractive optical elements in accordance with
various embodiments of the instant invention have been calculated to
yield about 2 degree for 16-phase levels of 1 μm pixel width. While
this beam steering is not as large, the NEF diffractive elements provide
the advantage of large aperture and thin form factor.

[0118]Another application of a polarization-selective hologram in
accordance with one embodiment of the instant invention is as a
beam-steering element in external cavity lasers. In external-cavity laser
systems, a linear polarizer is often used to preferentially select the
lasing polarization. The polarizer absorbs/reflects the unwanted
polarization and allows the required polarization to continue to build up
the round trip amplification before exiting the cavity. Organic
absorptive polarizers often lack the reliability requirements for high
power operation. A reflective type wiregrid based polarizer creates other
issues such as grid cleaning and metal layer absorption.

[0119]Referring to FIG. 26, an external-cavity solid-state laser system
1500 is shown to include a laser crystal 1501 having a front facet
coating 1502, a polarization-selective beam-steering device 1503 disposed
on a transparent substrate 1504, and a second harmonic generation crystal
1505 with an rear (exit) facet reflector 1506. The laser crystal 1501 is
typically doped with rare-earth metal elements, such as Nd:YAG (neodymium
doped yttrium aluminum garnet), Nd:YVO4 (neodymium doped yttrium
vanadate), etc., in order to produce an emission of the desired
wavelength. For example, the diode-pumped light maybe 808 nm whereas the
emission is 1064 nm. The second harmonic generation crystal, for example
KTP (potassium titanyl phosphate), is a bulk non-linear crystal which
converts the laser crystal emission into another wavelength (e.g, 532 nm
with the 1064 nm input light). The second harmonic generation may also be
obtained within the confined waveguide modes of periodically poling
lithium niobate. The polarization-selective grating 1503 allows a single
polarization of the fundamental frequency light to lase within the laser
cavity. The second harmonic light generated with the frequency doubler
crystal will then output the same polarization.

[0120]In operation, a diode-pump launches a light beam 1510 (e.g.,
λ=808 nm) into the laser crystal 1501 through the pump-light HT
(high transmission) coating 1502. This light is absorbed by the laser
crystal 1501, which causes an emission of the fundamental frequency light
(e.g., λ=1064 nm). The emitted light propagates forward as light
ray direction 1520 having a mixture of two orthogonal linear
polarizations which are parallel to the plane of drawing 1521 and
perpendicular to the plane of drawing 1522. The polarization-selective LC
grating 1503 allows the o-wave (e.g., linear polarization perpendicular
to the plane of drawing) to transmit through without deviation as beam
1530, while diffracting the e-wave (e.g., linear polarization parallel to
the plane of drawing) as beam 1540 having small deflection 1545. The
equivalent deflection angle in air, after the first pass through the LC
grating, θ1, is sin-1(λ/Λ). Upon reflection
from the high reflector 1506 at the fundament frequency light, the
deflected beam travels at -θ1 to the system axis as beam 1550.
This beam is again incident on the polarization-selective grating 1503,
and is transmitted through as beam 1560 which is steered further from the
system axis. This second pass beam maintains the linear polarization
parallel to the plane of drawing 1561, at an equivalent deflection angle
in air, sin(θ2)=sin(-θ1)-λ/Λ;
sin(θ2)=-2λ/Λ. The beam that has passed the LC
grating twice is reflected at the front facet reflector 1502 and
propagates as beam 1570 at -θ2 with respect to the system axis
towards the LC grating. This beam is again deflected a third time, giving
1580 and having a deflection angle 1585 given by
sin(θ3)=sin(-θ2)+λ/Λ;
sin(θ3)=3λ/Λ. It can be seen that the linear
polarization parallel to the plane of drawing is deflected away from the
optical system of the laser system with each transmission through the
polarization-selective LC grating. As a result, light having this
polarization is highly deviated from the gain segment of the laser
crystal such that a coherent lasing action is not permitted. The linear
polarization corresponding to the e-wave of the LC grating is suppressed
in the laser system and the second harmonic light generation at this
polarization is also suppressed. While the linear polarization parallel
to the plane of drawing is progressively deflected away from the optical
axis of the laser system, the linear polarization perpendicular to the
plane of drawing is reflected multiple times along the principal axis as
beam 1530. With each reflection of the front facet 1502 and the
rear-facet 1506 reflectors, the amplitude of the fundament frequency
light, polarized perpendicular to the plane of drawing is built up. Some
of this fundamental frequency light is converted into its second harmonic
light by the non-linear crystal 1505. The second harmonic light exits the
laser via a high-transmission rear-facet coating 1506.

[0121]Advantageously, the NEF polarization-selective LC hologram works as
a polarization discriminator in the external cavity laser by steering off
the unwanted linear polarization. The linear polarization that is
suppressed in the system can be chosen by the tilt-plane. The LC hologram
is fully flat and aids integrating, handling, and cleaning. In this
application, the functionality of the LC hologram is analogous to that of
a Rochon polarizer (e.g., where one beam of the first linear polarization
is undeflected while the orthogonal beam is diffracted slightly). For a
laser system amplification, a very slight angle deflection with each
round trip traversing is enough to decrease gain and result in no lasing
action for the polarization that is deflected. In addition, the NEF
polarization-selective LC grating has a large aperture and a relatively
thin form-factor. Note that the grating vector-plane selection is less of
importance in a radially-symmetric laser system.

[0122]In the above described embodiments, the NEF diffractive optical
elements have been single-layer LC grating films, which for example have
been used for aberration correction and holographic beam-splitting in OPU
systems and lasing polarization selection in external-cavity lasers. In
other embodiments, the NEF diffractive optical elements are formed from
more than one LC grating layer.

[0123]Referring to FIG. 27, a dual-stage device 1600 in accordance with
one embodiment of the instant invention includes two LC gratings similar
to that illustrated in FIG. 24 disposed in series. More specifically, the
compound device 1600 includes a first NEF diffractive optical element
1310 and a second NEF diffractive optical element 1610, which are
fabricated to be close to identical, and which are disposed such that the
deflection angles from the two stages are aligned with the same angle
sense. For example, in one embodiment the both the LC tilt-plane and the
grating vectors are the same in the each of the first and second stage LC
gratings.

[0124]In operation, a light beam 1320 including both linear polarizations
1321 is split by LC grating 1310 as o-wave 1330 and e-wave 1340. The
second LC grating 1610 placed after the first LC grating 1310 then steers
the e-wave a second time, giving a compound deflection angle
sin(θ)=2λ/Λ, where λ is the wavelength of
illumination and A is the grating pitch. The e-wave output 1640 from the
two-stage device has the linear polarization 1641 parallel to the plane
of drawing, with the deflection angle 1645. The unaffected linear
polarization perpendicular to the plane of drawing exits as beam 1630
with polarization 1631. This two-stage configuration may be useful if the
LC grating thickness cannot be configured to provide a single-stage
steering at the required angle of deflection.

[0125]Referring to FIG. 28, a dual-stage device 1700 in accordance with
another embodiment of the instant invention includes two of the LC
gratings illustrated in FIG. 24 disposed in series. More specifically,
the compound device 1700 includes a first NEF diffractive optical element
1310 and a second NEF diffractive optical element 1710, which are
fabricated to be close to identical, and which are disposed such that the
deflection angles from the two stages are aligned in opposite angle
sense. For example, in one embodiment the LC tilt-plane and grating
vectors are parallel in the first and second stage LC gratings, although
they do not necessarily coincide. Note, that although the tilt planes are
parallel in the two gratings, the gratings are disposed such that the
out-of-plane tilts are in opposite directions. For example, in one
embodiment the second LC grating 1710 is placed after the first LC
grating 1310 such that it is oriented with its azimuthal position rotated
by 180 degree, and such that the two LC gratings steer light beams with
oppositely signed angles.

[0126]In operation, a light beam 1320 including both linear polarizations
1321 is split by LC grating 1310 as o-wave 1330 and e-wave 1340. The
e-wave output from the first stage LC grating 1310 is deflected with an
angle sin(θ)=λ/Λ and this becomes the angle of
incidence in the second stage LC grating 1710. The e-wave output of the
second stage hologram now steers the incoming beam by
-sin-1(λ/Λ) which restores the input beam direction.
However, due to the propagation at angle θ between stage 1 and
stage 2 for a given distance l 1750 the beam undergoes a lateral
translation Δx. This lateral translation 1751 is approximately
given by Δx=l*tan(θ) in air. Accordingly, this two-stage
device 1700 functions as a beam walk-off element or a beam displacer.

[0127]Accordingly, another application of a polarization-selective
hologram in accordance with one embodiment of the instant invention is as
a beam displacer in an optical circulator, isolator, optical low-pass
filter, etc. Advantageously, the polarization-selective hologram, used as
a walk-off device with parallel ordinary-ray (o-ray) and
extraordinary-ray (e-ray) outputs, is fabricated by cascading two similar
gratings. In particular, a first linear grating (1D) sets up a
high-efficiency single-order grating replay such that the exiting beam
propagates forwards at a characteristic deflection angle until a second,
inverse signed angle steering 1D grating corrects for the non-normal beam
angle. For a given grating geometry and depending on the gap between the
two hologram stages, the lateral offset between the parallel o-ray and
e-ray is set accordingly.

[0128]Referring to FIG. 29, a dual-stage device 1800 in accordance with
another embodiment of the instant invention includes two of the LC
gratings illustrated in FIG. 24 disposed in series. More specifically,
the compound device 1800 includes a first NEF diffractive optical element
1310 and a second NEF diffractive optical element 1810, which are
fabricated to be close to identical, and which are disposed such that the
LC tilt planes of the two LC grating stages are aligned perpendicular,
and such that the two LC hologram stages act on orthogonal linear
polarizations. More specifically, the second stage LC grating 1810 is
arranged to have its grating vector plane parallel to that of 1310, but
with the LC tilt plane at perpendicular plane to that of 1310. The LC
indicatrices shown are projections onto the plane of drawing. The second
stage LC grating is also configured to steer to the opposite signed angle
as the first stage LC grating. As a result of this configuration, the
o-wave 1330 and e-wave 1340 outputs from the first LC hologram exit the
second LC grating as e-wave 1840 and o-wave 1830, respectively. The
e-wave 1840 is steered through an angle -sin-1(λ/Λ)
whereas the o-wave 1830 output is unaffected (exit at the original
steering angle sin-1(λ/Λ)).

[0129]Referring to FIG. 30, a dual-stage device 1900 in accordance with
another embodiment of the instant invention includes two of the LC
gratings illustrated in FIG. 24 disposed in series. More specifically,
the compound device 1900 includes a first NEF diffractive optical element
1310 and a second NEF diffractive optical element 1910, which are
fabricated to be close to identical, and which are disposed such that the
LC tilt planes of the two LC grating stages are aligned perpendicular,
and such that both linear polarizations inputs to the device are
beam-steered. More specifically, the second stage LC grating 1910 is
arranged to have its grating vector plane parallel to that of 1310, but
with the LC tilt plane at perpendicular plane to that of 1310. The LC
indicatrices shown are projections onto the plane of drawing. The second
stage LC grating is also configured to steer to the same signed angle as
the first stage LC grating. As a result of this configuration, the o-wave
1330 and e-wave 1340 outputs from the first LC hologram exit the second
LC grating as e-wave 1940 and o-wave 1930, respectively. The e-wave 1840
is steered through an angle sin-1(λ/Λ) whereas the
o-wave 1830 output is unaffected (exit at the original steering angle
sin-1(λ/Λ)). Both o- and e-waves exit the compound
device parallel. The unique functionality here is that this compound
grating is no longer polarization-selective. Bar the small lateral offset
due to the thickness of the LC gratings (say several microns), any
polarization input is steered by angle θ to the optical axis. The
two substrates in the depicted device 1319 and 1919 may be omitted by
coating both the LC grating layers 1310 and 1910 successively on a single
substrate.

[0130]Each of the four dual-stage configurations 1600, 1700, 1800 and 1900
discussed above, the devices have been configured to have parallel
grating vectors in stage one and stage two. In other embodiments, a
dual-stage configuration having arbitrary first stage and second stage
steering planes (dictated by the grating vectors) is provided. In this
case, the LC tilt planes in the first and second gratings will be either
parallel or perpendicular to accept both linear polarization inputs.

[0131]The two-stage LC holograms have been simulated with an RCWA
[rigorous coupled-wave analysis, GSolver by Grating Solver Development
Company, Allen, Tex., version 4.20b] program at λ=550 nm, by
representing the LC grating as non-polarization-selective air/dielectric
blazed grating having 16 phase pixels of 1 μm width each. The results
are shown in FIG. 31. A right blaze is a stairs-steps like phase ramp
with the right side of a single grating pitch having a longer optical
path length when the observer is viewing the beam head-on. This blazed
grating steers the beam to the first order, which is located to the right
of the zeroth order, as shown in plot (a) of FIG. 31. In this case, the
DE approaches 92%, without AR coating on the air/1.5 index grating. In
plot (b) of FIG. 31 a first right blazed air/1.5 index grating is
followed by a second right blazed air/1.5 index grating. This dual-stage
grating steers the output light to twice the spatial frequency as
compared to a single grating (plotted as order of 2). In the simulation,
neither grating was AR coated and the inter-grating layer had an index of
1.5 and 220 μm physical thickness. The beam displacer is illustrated
by results in plot (c) of FIG. 31. The compound grating had a first
left-blazed grating nearer to the incidence, followed by a second right
blazed grating adjacent to the substrate. The two gratings are separated
by an inter-grating layer of 1.5 index and 220 μm physical thickness.
Both gratings had identical 16 phase levels, forming a ramp over 16-μm
grating pitch length. The result shows that the steering angle imposed by
the first stage grating is corrected by the second stage grating. The
output beams are co-linear but are spatially offset by a certain amount
(not shown in diffraction simulation). Both dual-stage simulation
examples produced about 82% of main order efficiency.

[0132]Referring to FIG. 32 the first LC gratings 1310 and second LC
gratings 1710 are shown disposed on opposite sides of a single substrate
2010, respectively. Note that the NEF diffractive optical element 2000 is
functionally equivalent to the compound NEF diffractive optical element
1700. In these figures, like numerals are used to defined like elements.
The transparent substrate 2010 supports the LC grating layers and
functions as an inter-grating layer. Using the 2-degree steering example
described previously (e.g., with 16-pixel grating at 1 μm pixel pitch)
and assuming a 1.5 index for the inter-grating layer 2010, a ˜220
μm inter-grating layer will give rise to approximately 5 μm of beam
displacement. The exit beams, polarized parallel and perpendicular to the
grating vector, are parallel headed. This walk-off of ˜5 μm
meets the requirement of optical low-pass filter applications. In digital
imaging systems, an anti-aliasing technique is to utilize beam walk-off,
to ensure that a minimum image spot size is focused onto the electronic
CCD/CMOS array backplane. The walk-off is typically implemented with
45-degree cut uniaxial crystal plates of suitable thickness. Crystal
plates are expensive to manufacture. Alternatively, spin-coated
homogeneous LC films, aligned at 45-degree can be used to provide a
suitable walk-off (e.g., see U.S. Pat. No. 7,088,510). However, the
difficulty associated with the fabrication of thick LC layers (tens of
microns) at the required 45-degree tilt makes a homogenous tilt LC film
impractical. In comparison, the two-layer LC grating 2000 accomplishes
the beam displacement by first providing a beam steering function in the
first LC grating, allowing the deflected beam to accumulate spatial
offset by an inter-grating layer and finally correcting the beam angle by
a second LC grating.

[0133]Referring to FIG. 33, there is shown another embodiment of the
instant invention, wherein the LC gratings are separated by a deposited
inter-grating layer, and are provided on a single-side of a substrate.
More specifically, the device 2050 includes a transparent substrate 1719,
onto which a first LC grating 1310 and a second LC grating 1710 are
provided, wherein the first and second LC grating layers are separated by
an inter-grating layer 2010. Like numerals have the same definitions as
those in FIGS. 28 and 32. The exiting beams are polarized orthogonally
and are co-linear. The beam separation at the exit is given by,

Δx=l*tan(sin-1(λ/(nΛ))), (9)

where l is the layer thickness of the inter-grating layer having an index
of refraction n, λ is the wavelength of illumination, and Λ
is the grating pitch.

[0134]Another application of the NEF diffractive optical elements of the
instant invention is as a two-dimensional (2D) walk-off element in an
optical low pass filter (OLPF). For example in one embodiment, multiple
stages of a walk-off device similar to that shown in FIG. 28 are cascaded
to form a OLPF used to cut off high spatial frequency image components in
digital imaging systems. Referring to FIG. 34, the 2D walk-off device
2100 includes a first walk-off LC grating device 2000, a second
orthogonal-axis walk-off LC grating device 2110, and a polarization
scrambler 2120. For an input wave 1320 having two orthogonal linear
polarizations 1321, the first walk-off LC grating device 2000 displaces
the e-wave beam 1740 by a predetermined amount 1751 relative to the
unaffected o-wave beam 1730. The two co-linear beams (parallel in
direction of propagation) at orthogonal linear polarizations are then
scrambled by the polarization scrambler 2120 to yield both orthogonal
linear polarizations for each beam. In one embodiment, the polarization
scrambler 2120 is a retarder element, such as a quarter-waveplate. The
two beams, which include linear polarizations both parallel and
perpendicular to the plane of drawing, propagate to the second walk-off
LC grating device 2110. The grating vector for the second LC grating
device 2110 is arranged orthogonal to the grating vector of the first
grating device 2000. By this arrangement, the output of the first grating
device 2000 is displaced along the plane of drawing while the output of
the second grating device 2110 is displaced perpendicular to the plane of
drawing.

[0135]When the polarization scrambler 2120 is a quarter-waveplate, the
fast/slow axis of the quarter-waveplate (QWP) is aligned typically at
±45 degree with respect to the plane of drawing. The two beams 1730
and 1740 exiting the first walk-off LC grating device are converted to
circular polarization by the QWP (i.e. there is equal amount of linear
polarizations along any two orthogonal directions). It may be common to
choose the tilt-plane to be either parallel (shown in FIG. 34) or
orthogonal (not shown) to the grating vector for the second walk-off LC
grating device. Approximately half of each beam power is displaced into
the plane of drawing by the second walk-off LC grating device. This set
of two beams is shown as 2133 and 2134 in FIG. 34. They are polarized
parallel to the tilt-plane of the second walk-off LC grating device. The
remaining two beams 2131 and 2132, which were unaffected, are polarized
perpendicular to the tilt-plane of the second walk-off LC grating device.
Accordingly, the 2D OLPF produces four beam spots for each beam input
arrangement in a square grid (or rectangular grid if the quantum of
displacement for first stage is not the same as the second stage).

[0136]The beam walk-off pattern is shown as plot (a) in FIG. 35. The first
stage walk-off displaces a single input beam into two approximately equal
intensity spots, as indicated by the solid arrow. Prior to the second
stage walk-off, the polarizations of both beams are scrambled. A second
stage walk-off along an orthogonal axis then results in four beam spots
distributed at four adjacent CCD/CMOS pixels.

[0137]In case of walk-off via 45-degree cut crystal plate and without the
use of a polarization scrambler, the second stage walk-off may be
arranged to have the e-wave axis at ±45 degree with respect to the
first walk-off stage output. Each first stage walk-off output beam is
resolved into half e-wave and half o-wave. The e-wave is further
displaced along the ±45 degree diagonal, resulting a diamond shape
walk-off pattern (e.g., see plot (b) in FIG. 35).

[0138]In the case of the walk-off via polarization-selective LC gratings,
the polarization scrambler stage may be omitted without sacrificing the
ideal square walk-off pattern. The plan view of the two-stage walk-off
OPLF with a quarter-waveplate polarization scrambler is depicted in FIG.
36. In (a) the walk-off LC grating device 2000 is shown with a horizontal
grating vector. The LC indicatrix projections onto the plane of drawing
are shown as 2001 and 2002 for the first layer and the second layer
within the first walk-off device. The quarter-waveplate 2120 is shown
with a slow-axis 2121 aligned at 45 degree with respect to the grating
vector of the first walk-off device (e.g., see diagram (b)). The second
walk-off grating device 2110 has its grating vector aligned vertically
(e.g., perpendicular to the first grating vector). As was stated
previously, the tilt-plane of the second grating device can be chosen
arbitrarily since the polarization scrambler results in circular
polarization input to the second walk-off grating device. The diagram in
(c) illustrates tilt-plane aligned along the second grating vector. The
LC indicatrix projections onto the plane of drawing are labelled 2111 and
2112. The second walk-off grating device displaces the beam in the
vertical direction for the fraction of power aligned at vertical
polarization.

[0139]As discussed above, it is also possible to configure the OPLF
without the intermediate polarization scrambler. This scheme is
illustrated with reference to FIG. 37. The first walk-off grating device
is shown in (a) having a first grating vector in the horizontal plane,
similar to that shown in (a) of FIG. 36. The two beams exiting the first
walk-off grating device are polarized parallel 2006 and perpendicular
2007 to the plane of drawing. In order to obtain approximately equal
e-wave and o-wave power fraction from each beam without polarization
scrambling, the tilt-plane of the second walk-off grating device has to
be aligned ±45 degree with respect to the first grating vector. The LC
indicatrix projections of the first and second grating layers within the
second walk-off grating device are shown as 2113 and 2114. The input to
the second walk-off grating device having a 90 degree second grating
vector alignment is shown as 2008 and 2009, each of which has
approximately half beam power along the tilt-plane. The e-wave fractions
are displaced vertically (e.g., 90 degree azimuth direction) whereas the
o-wave fractions are unaffected. The overall device produces four beam
spots for each incoming beam spot, with two-stage walk-off grating
devices and without a polarization scrambler.

[0140]In the embodiments of the instant invention described above, the NEF
polarization-selective diffractive optical element provides a thin
hologram element, operating within the paraxial diffraction limit, by
judicially arranging the LC out-of-plane tilt across a transverse spatial
coordinate in a predetermined manner. The resultant NEF thin hologram has
the LC directors aligned homogeneously along a given azimuthal plane. The
plane containing the LC director distribution is also the tilt plane.
Only light rays polarized along the tilt plane are affected by the
variable amount of retardance encoded continuously or in a pixelated
manner. The variable amount of retardance is a manifestation of variable
optical path length modulation as a function of transverse spatial
coordinate. Conversely, light rays polarized along a direction orthogonal
to the tilt-plane sees only the ordinary index of refraction regardless
of LC director tilt. The variable optical path length modulation is
absent and this orthogonal polarization essentially experiences a
zeroth-order grating.

[0141]Advantageously, the polarization-selectivity of these NEF thin
holograms is exploited in various applications that use linearly
polarized light. Some applications related to the
polarization-selectivity have been outlined, which include aberration
compensation and holographic beam splitting in OPU systems, beam steering
based polarization-selection in an external-cavity solid-state laser, and
beam walk-off device in optical low-pass filter. Obviously, more
applications can be identified with either a single-layer LC hologram or
multiple-layer or multiple-stage LC holograms which are
polarization-selective. The polarization selectivity is inherent in the
LC device with a homogeneous azimuthal orientation. However, in some
applications, the selectivity is deliberately turned off, for example by
coupling two LC hologram layers with orthogonal tilt plane orientations.
Further advantageously, the fabrication technique used to create the NEF
diffractive optical elements allows for multi-level phase-only holograms
to be recorded such that high diffraction efficiencies are obtained.

[0142]Yet another application of the NEF diffractive optical elements is
as a variable magnitude birefringent compensator. For example, consider
the prior art Babinet-Soleil compensator, which includes two birefringent
crystal wedges (e.g., quartz) disposed adjacent to another birefringent
plate of the orthogonal birefringent axis alignment. By mechanically
translating one of the wedges, a variable amount of retardance is
presented to the narrow-diameter probing beam.

[0143]A conventional Babinet-Soleil compensator is illustrated in FIG. 38.
This variable-retardance compensator 2200 includes a first homogeneously
aligned A-plate 2201 coupled to another birefringent plate made of two
birefringent wedges 2202 and 2203. The A-plate 2201 has its optic axis
aligned parallel to the striped direction. The birefringent wedges
2202/2203 are typically cut from crystalline material and are aligned
with their optic axes parallel to the striped direction. In other words,
the optical axes of the wedges are parallel to each other, but are
orthogonal to the optic axis of the first birefringent plate. The top
birefringent wedge 2203, which has its angled-facet facing the
angled-facet of the other wedge 2203, can be translated mechanically in a
direction parallel to the optic axis of the first birefringent plate
(i.e., along 2204). This lateral translation results in an effective
retardance provided by the combined two wedges. This retardance magnitude
is then offset from a second retardance magnitude provided by the first
birefringent plate. The retardance difference is the effective retardance
as seen by the light input 2220. This device configuration is similar to
a multiple-order waveplate, with the required retardance provided by the
difference in retardance realized in each of the two crossed axes
retarders. In the case of the Babinet-Soleil compensator 2200, the net
amount of retardance is adjustable by lateral translation of the top-most
wedge.

[0144]In accordance with an embodiment of the instant invention, a NEF
diffractive optical element is used as a variable magnitude birefringent
compensator. In particular, the LC out-of-plane director distribution is
patterned to provide a precise and accurate variable magnitude
birefringence. Referring to FIG. 39, the variable retarder 2300 includes
a single layer of LCP, wherein the LC director is distributed in some
predetermined manner in such a way that the resultant retardance along a
given transverse spatial coordinate is varied in the required manner
(e.g., linear versus X-coordinate). This monolithic variable retarder
2300 is shown with several segments of LC director distribution such as
C-plate 2301, O-plate 2302 and A-plate 2303. The A-plate segment presents
the largest amount of retardance relative to the O-plate and/or C-plate
segments for a given physical LC thickness. If a linear retardance
profile is desired versus transverse spatial coordinate, the LC tilt
profile is tailored in a non-linear manner. To obtain a different amount
of retardance for a given light input location 2320, the entire variable
retarder is translated by mechanical actuation means 2304 such that a
different spatial region is aligned to the input beam. A wide-band
variable retarder according to the present invention is feasible, as in
the prior-art crystal plate scheme. For example, a variable retarder
covering λ=400 nm to 1600 nm with up to 1 wave of retardance at the
longest wavelength can be configured with a single layer LC film having a
continuous LC director variable from C-plate to A-plate. The LC film is
assumed to yield about 0.1 birefringence at the long wavelength edge.
Hence, the LC film is about 16 μm thick. The short wavelength will see
more than 1 wave of retardance due to the normal material index
dispersion within this band.

[0145]Advantageously, this tunable retarder, which is obtained by
continuously splaying the LC out-of-plane tilt as a function of linear
position while maintaining a given azimuthal direction, provides variable
retardance up to small multiples of lambda with appropriate selection of
the device thickness.

[0146]Further advantageously, the large substrate handling capability of a
non-etched, flat retarder technology allows for multiple retarder
magnitude ramps to be patterned and exposed onto a large format
substrate. At the wafer level, a grating/hologram type coarse resolution
pattern is obtained. Each "period" within the large wafer substrate can
be diced into a discrete variable retarder at singulation stage. In
general, the slow/fast-axis of the monolithic variable retarder will be
anchored homogeneously along a required azimuth, such as ±45 degree
versus the rectangular geometry of the retarder. Although polarization
selectivity is inherent this NEF diffractive optical element due to the
homogeneous azimuthal orientation, in use, the probing beam typically
will be small relative to the dimension of the variable retarder (e.g., 1
mm beam size versus 10 mm end-to-end translation range), such that the
variable retarder will not necessarily function as a
polarization-selective diffractive optical element.

[0147]In each of the above-described embodiments, the fabrication
technique used to create the NEF diffractive optical elements only
requires a single substrate, and thus produces thinner passive optical
elements that are relatively inexpensive, and that are suitable for a
wide range of applications. In comparison, prior art references U.S. Pat.
No. 7,375,784 and U.S. Pat. No. 6,304,312 both require two transparent
substrates, which cooperate to induce alignment of the liquid crystal in
the relatively thick liquid crystal cell. In addition, these prior art
fabrication techniques are not compatible with providing multi-level
phase-only holograms. In contrast, the instant invention provides
multi-level phase-only holograms having features that are 1 μm or
smaller (e.g., when an array of variable optical path regions are
provided in a predetermined manner). Notably, the fabrication techniques
used to for the NEF diffractive optical elements do not require the
traditional masked and etched processes that provides a surface relief
structure (SRS). The fabrication techniques for the present invention
also do not require the fabrication of Liquid Crystal cells as an
intermediate step and no transparent electrodes for applying electrical
pulses for LC alignment are needed. In addition, unlike absorption-based
(e.g., intensity modulation) holograms, the resultant phase-only
holograms can be made lossless. These passive phase-only LC holograms are
also expected to yield higher diffraction efficiencies due to better
control of the pixel-fill duty cycle ratio when compared to the actively
switched LC hologram, where the SLM pixel array requires row/column
addressing lines and pixel addressing circuitry.

[0148]Of course, the above embodiments have been provided as examples
only. It will be appreciated by those of ordinary skill in the art that
various modifications, alternate configurations, and/or equivalents will
be employed without departing from the spirit and scope of the invention.
For example, various periodic and non-periodic patterns can be used to
form the polarization-selective phase holograms (e.g., used for beam
steering). In some embodiments, these polarization-selective phase
holograms have a pixelated phase profile. In other embodiments, the
polarization selective phase holograms have a continuous phase profile.
Accordingly, the scope of the invention is therefore intended to be
limited solely by the scope of the appended claims.